The Language of Chemistry

The formula of a compound represents

1. an atom
2. a particle
3. a molecule
4. a combination

Answer

a molecule

Reason — The formula of a compound employs symbols to denote the molecule of a compound.
An atom is the smallest unit of an element that retains its chemical properties (like H, O, Na). A formula of a compound does not represent a single atom, because compounds are made of two or more different atoms chemically combined. A particle is a very broad term. It includes atom, molecule, ion.<>

The correct formula of aluminium oxide is

1. AlO₃
2. AlO₂
3. Al₂O₃
4. Al₃O₂

Answer

Al₂O₃

Reason — The formula of aluminium oxide can be determined as below:

$\text{Al}^{3+} \space \text{O}^{2-} \\[1em] \overset{\phantom{3}{3}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{2}{2}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{O}} \\[1em] \text{Al}_2\text{O}_3$

The valency of lead in lead oxide (PbO) is

1. one
2. two
3. three
4. four

Answer

two

Reason — The valency of lead in lead oxide can be determined from the formula PbO as below:

$\text{PbO} = \underset{\phantom{1}{1}}{\text{Pb}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{1}{\text{O}} \Rightarrow \overset{\phantom{1}{1}}{\text{Pb}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{1}{\text{O}} \\[0.5em]$

But valency of O is 2. Multiplying by 2, we get:

$\overset{{1 \times 2}}{\text{Pb}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2 \times 1}{\text{O}} \Rightarrow \overset{\phantom{2}{2}}{\text{Pb}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{O}} \\[0.5em]$

Therefore, valency of Lead is 2.

Which of the following elements has a molecular mass that is four times its atomic mass?

1. Oxygen
2. Neon
3. Phosphorus
4. Sulphur

Answer

Phosphorus

Reason

Atomic mass of P = 31 μ

Molecular form = P₄ ⟶ Molecular mass = 31 × 4 = 124 μ

124 μ is 4 times the atomic mass

Hence, phosphorus has molecular mass four times its atomic mass.
Oxygen is a diatomic molecule. Neon exists as single atoms. Sulphur exists as S₈ molecules.

Match the following : (Refer common names in the beginning of the book)

Answer

Select the basic and acidic radicals in the following compounds.

(a) MgSO₄

(b) (NH₄)₂SO₄

(c) Al₂(SO₄)₃

(d) ZnCO₃

(e) Mg(OH)₂

Answer

The basic and acidic radicals in the compounds are given in the table below:

Give the formula and valency of :

(a) aluminate

(b) chromate

(c) aluminium

(d) cupric

Answer

The formula and valency are given in the table below:

What do the following symbols stand for ?

(a) H

(b) H₂

(c) 2H

(d) 2H₂

Answer

(a) H → One atom of Hydrogen.

(b) H₂ → One molecule of Hydrogen.

(c) 2H → Two atoms of Hydrogen.

(d) 2H₂ → Two molecules of Hydrogen.

Write the chemical names of the following compounds :

(a) Ca₃(PO₄)₂

(b) K₂CO₃

(c) K₂MnO₄

(d) Mn₃(BO₃)₂

(e) Mg(HCO₃)₂

(f) Na₄Fe(CN)₆

(g) Ba(ClO₃)₂

(h) Ag₂SO₃

(i) (CH₃COO)₂Pb

(j) Na₂SiO₃

Answer

(a) Ca₃(PO₄)₂ → Calcium Phosphate

(b) K₂CO₃ → Potassium Carbonate

(c) K₂MnO₄ → Potassium Manganate

(d) Mn₃(BO₃)₂ → Manganese (II) borate

(e) Mg(HCO₃)₂ → Magnesium Hydrogen Carbonate

(f) Na₄Fe(CN)₆ → Sodium Ferrocyanide

(g) Ba(ClO₃)₂ → Barium Chlorate

(h) Ag₂SO₃ → Silver Sulphite

(i) (CH₃COO)₂Pb → Lead Acetate

(j) Na₂SiO₃ → Sodium Silicate

Write chemical formulae of the sulphates of Aluminium, Ammonium and Zinc.

Answer

(a) Chemical formula of Aluminium Sulphate:

$\text{Al}^{3+} \space \text{SO}_4^{2-} \\[1em] \overset{\phantom{3}{3}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{SO}_4} \\[1em] \text{Al}_2\text{(SO}_4)_3$

∴ Chemical formula of Aluminium Sulphate is Al₂(SO₄)₃

(b) Chemical formula of Ammonium Sulphate:

$\text{NH}_4^{+} \space \text{SO}_4^{2-} \\[1em] \overset{\phantom{3}{1}}{\text{NH}_4} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{NH}_4} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{1}}{\text{SO}_4} \\[1em] \text{(NH}_4)_2\text{SO}_4$

∴ Chemical formula of Ammonium Sulphate is (NH₄)₂SO₄

(c) Chemical formula of Zinc Sulphate:

$\text{Zn}^{2+} \space \text{SO}_4^{2-} \\[1em] \text{Dividing by H.C.F. it becomes} \\[1em] \text{Zn}^{1+} \space \text{SO}_4^{1-}\\[1em] \overset{\phantom{22}{1}}{\text{Zn}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{1}}{\text{Zn}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{1}}{\text{SO}_4} \\[1em] \text{ZnSO}_4$

∴ Chemical formula of Zinc Sulphate is ZnSO₄

The valency of an element A is 3 and that of element B is 2. Write the formula of the compound formed by the combination of A and B.

Answer

Formula of the compound formed by the combination of A and B:

$\overset{\phantom{3}{3}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{B}} \Rightarrow \underset{\phantom{2}{2}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{B}} \\[1em] \text{A}_2\text{B}_3$

∴ Formula of the compound is A₂B₃

Give the names of the following compounds.

(a) KClO

(b) KClO₂

(c) KClO₃

(d) KClO₄

Answer

(a) KClO → Potassium hypochlorite

(b) KClO₂ → Potassium Chlorite

(c) KClO₃ → Potassium Chlorate

(d) KClO₄ → Potassium Perchlorate

The formula of the sulphate of an element M is M₂(SO₄)₃. Write the formula of its

(a) Chloride

(b) Oxide

(c) Phosphate

(d) Acetate

Answer

Given, formula of sulphate is M₂(SO₄)₃.

$\text{M}_2(\text{SO}_4)_3 = \underset{\phantom{2}{2}}{\text{M}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{3}{\text{SO}_4} \Rightarrow \overset{\phantom{3}{3}}{\text{M}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{SO}_4} \\[0.5em]$

∴ Valency of M is 3.

(a) Formula of Chloride:

$\text{M}^{3+} \space \text{Cl}^{1-} \\[1em] \overset{\phantom{3}{3}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{Cl}} \\[1em] \text{M}\text{Cl}_3$

∴ Formula of chloride of element M is MCl₃

(b) Formula of Oxide:

$\text{M}^{3+} \space \text{O}^{2-} \\[1em] \overset{\phantom{3}{3}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{2}{2}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{O}} \\[1em] \text{M}_2\text{O}_3$

∴ Formula of oxide of element M is M₂O₃

(c) Formula of Phosphate:

$\text{M}^{3+} \space \text{PO}_4^{3-} \\[1em] \text{Dividing by H.C.F. it becomes} \\[1em] \text{M}^{1+} \space \text{PO}_4^{1-} \\[1em] \overset{\phantom{3}{1}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{1}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{1}}{\text{PO}_4} \\[1em] \text{MPO}_4$

∴ Formula of phosphate of element M is MPO₄

(d) Formula of Acetate:

$\text{M}^{3+} \space \text{CH}_3\text{COO}^{1-} \\[1em] \overset{\phantom{3}{3}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \space \overset{\phantom{CHCOOO}1}{\text{CH}_3\text{COO}} \Rightarrow \underset{\phantom{1}{1}}{\text{M}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{CH}_3\text{COO}} \\[1em] \text{M}\text{(CH}_3\text{COO})_3$

∴ Formula of acetate of element M is M(CH₃COO)₃

What is a symbol of an element ? What information does it convey ?

Answer

A symbol is a short form that stands for the atom of a specific element or the abbreviation used for the name of an element.

A symbol conveys the following information:

1. It represents the name of the element.
2. It represents one atom of the element.
3. It represents a definite mass of the element (equal to atomic mass expressed in grams).

Why is the symbol S for sulphur, but Na for sodium and Si for silicon ?

Answer

When the first letter of more than one element is the same the elements are denoted by two letters. Sulphur, Sodium and Silicon all have the first letter as S. Therefore, only sulphur is denoted by S, Silicon is denoted by two letter Si, Sodium is denoted by two letter Na taken from its latin name Natrium.

Write the full form of IUPAC.

Answer

Full form of IUPAC is International Union of Pure and Applied Chemistry.

Name the elements represented by the following symbols :

Au, Pb, Sn, Hg.

Answer

The elements represented by the following symbols are:

Au → Gold

Pb → Lead

Sn → Tin

Hg → Mercury

If the symbol for Cobalt, Co, were written as CO, what would be wrong with it ?

Answer

When writing symbols, we need to be careful about the case of the letters in the symbol. CO means the compound Carbon Monoxide not Cobalt.

Explain the terms 'valency' and 'variable valency'.

Answer

Valency is the combining capacity of an atom or of a radical. The valency of an element or of a radical is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.
Variable valency is the ability of certain elements to have more than one valency or different combining capacities. An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e., there is a loss of electrons from the penultimate shell too. Such element is said to exhibit variable valency.

How are the elements with variable valency named ? Explain with an example.

Answer

If an element exhibits two different positive valencies, then the suffix "ous" is used for the lower valency and the suffix "ic" is used for the higher valency. Modern chemists use Roman numerals in place of these trivial names.
For example, Iron can exhibit a valency of +2 or +3. The ion with valency +2 is named as Ferrous ion (Fe²⁺) and with valency +3 is named as Ferric ion (Fe³⁺). Its oxides will be named as Ferrous oxide or Iron (II) oxide [FeO] and Ferric oxide or Iron (III) oxide [Fe₂O₃].

(a) What is a chemical formula ?

(b) What is the significance of a formula ? Give an example to illustrate.

Answer

(a) A chemical formula also known as molecular formula employs symbols to denote the molecule of an element or of a compound.

(b) The chemical/molecular formula of a compound has quantitative significance. It represents:

1. both the molecule and the molecular mass of the compound.
2. the respective numbers of different atoms present in one molecule of the compound.
3. the ratios of the respective masses of the elements present in the compound.

For example, the formula CO₂ means that:

1. the molecular formula of carbon dioxide is CO₂.
2. each molecule contains one carbon atom joined by chemical bonds with two oxygen atoms.
3. the molecular mass of carbon dioxide is 44, given that the atomic mass of carbon is 12 and that of oxygen is 16.

What do you understand by the following terms ?

(a) Acid radical

(b) Basic radical

Answer

(a) An Acid radical is the radical that remains after an acidic molecule loses a hydrogen ion (H⁺). Acid radicals typically have a negative charge. They are also called electronegative radicals or anions.

(b) A Basic radical is the radical that remains after a base molecule loses a hydroxyl ion (OH^-). Basic radicals typically have a positive charge. They are also called electropositive radicals or cations.

Write the basic radicals and acidic radicals of the following and then write the chemical formulae of these compounds.

(a) Barium sulphate

(b) Bismuth nitrate

(c) Calcium bromide

(d) Ferrous sulphide

(e) Chromium sulphate

(f) Calcium silicate

(g) Stannic oxide

(h) Sodium zincate

(i) Magnesium phosphate

(j) Sodium thiosulphate

(k) Stannic phosphate

(l) Nickel bisulphate

(m) Potassium manganate

(n) Potassium ferrocyanide

Answer

Give the names of the elements and number of atoms of those elements, present in the following compounds.

(a) Sodium sulphate

(b) Quick lime

(c) Baking soda

(d) Ammonia

(e) Ammonium dichromate

Answer

(a) Sodium sulphate
     Formula: Na₂SO₄
   ∴ It contains 2 atoms of Sodium (Na), 1 atom of Sulphur (S) and 4 atoms of Oxygen (O).

(b) Quick lime
     Formula: CaO
   ∴ It contains 1 atom of Calcium (Ca) and 1 atom of Oxygen (O).

(c) Baking soda
     Formula: NaHCO₃
   ∴ It contains 1 atom of Sodium (Na), 1 atom of Hydrogen (H), 1 atom of Carbon (C) and 3 atoms of Oxygen (O).

(d) Ammonia
     Formula: NH₃
   ∴ It contains 1 atom of Nitrogen (N) and 3 atoms of Hydrogen (H).

(e) Ammonium dichromate
     Formula: (NH₄)₂Cr₂O₇
   ∴ It contains 2 atoms of Nitrogen (N), 8 atoms of Hydrogen (H), 2 atoms of Chromium (Cr) and 7 atoms of Oxygen (O).

The valency of an element A is 4. Write the formula of its:

(a) oxide

(b) nitrate

(c) phosphate

Answer

(a) Chemical formula of Oxide of A:

$\text{A}^{4+} \space \text{O}^{2-} \\[1em] \text{Dividing by H.C.F. it becomes} \\[1em] \text{A}^{2+} \space \text{O}^{1-}\\[1em] \overset{{2}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{O}} \Rightarrow \underset{\phantom{2}{1}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{2}}{\text{O}} \\[1em] \text{AO}_2$

∴ Chemical formula of Oxide of A is AO₂

(b) Chemical formula of Nitrate of A:

$\text{A}^{4+} \space \text{NO}_3^{1-} \\[1em] \overset{{4}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{4}}{\text{NO}_3} \\[1em] \text{A(NO}_3)_4$

∴ Chemical formula of Nitrate of A is A(NO₃)₄

(c) Chemical formula of Phosphate of A:

$\text{A}^{4+} \space \text{PO}_4^{3-} \\[1em] \overset{{4}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{A}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{4}}{\text{PO}_4} \\[1em] \text{A}_3(\text{PO}_4)_4$

∴ Chemical formula of Phosphate of A is A₃(PO₄)₄

Identify the cations (electropositive ions) and anions (electronegative ions) in the following compounds:

(a) CH₃COONa

(b) NH₄Cl

(c) PbCl₂

(d) MgO

Answer

(a) CH₃COONa (Sodium acetate)
     Cation: Na⁺ (Sodium ion)
     Anion: CH₃COO^- (Acetate ion)

(b) NH₄Cl (Ammonium chloride)
     Cation: NH₄⁺ (Ammonium ion)
     Anion: Cl^- (Chloride ion)

(c) PbCl₂ (Lead(II) chloride)
     Cation: Pb²⁺ (Lead(II) ion)
     Anion: Cl^- (Chloride ion)

(d) MgO (Magnesium oxide)
     Cation: Mg²⁺ (Magnesium ion)
     Anion: O^2- (Oxide ion)

Depending on the number of oxygen atoms in the anions, the names of some acids and their formulae are given. Write the formula of their corresponding salts.

Answer

Give an example of a divalent anion and a trivalent cation. Write the formula of the compound formed by their combination.

Answer

Aluminium is trivalent cation with a valency of 3 → Al³⁺

Sulphate is divalent anion with a valency of 2 → SO₄^2-

Formula:

$\text{Al}^{3+} \space \text{SO}_4^{2-} \\[1em] \overset{3}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{2}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \underset{3}{\text{SO}_4} \\[1em] \text{Al}_2(\text{SO}_4)_3$

Hence, the formula is : Al₂(SO₄)₃

Write the Latin names of sodium, mercury and iron.

Answer

State the number of atoms present in each of the following:

(a) PO₄^3-

(b) P₂O₅

(c) H₂SO₄

(d) SO₄^-2

Answer

(a) PO₄^3-

P = 1 atom

O = 4 atoms

Total atoms = 1 + 4 = 5 atoms

(b) P₂O₅

P = 2 atoms

O = 5 atoms

Total atoms = 2 + 5 = 7 atoms

(c) H₂SO₄

H = 2 atoms

S = 1 atom

O = 4 atoms

Total atoms = 2 + 1 + 4 = 7 atoms

(d) SO₄^-2

S = 1 atom

O = 4 atoms

Total atoms = 1 + 4 = 5 atoms

What is a chemical equation ? Why it is necessary to balance it ?

Answer

A chemical equation is the symbolic representation of a chemical reaction using the symbols and formulae of the substances involved in the reaction.

An equation must be balanced in order to comply with the "Law of Conservation of Matter", which states that matter is neither created nor destroyed in the course of a chemical reaction. An unbalanced equation would imply that atoms have been created or destroyed.

State the information conveyed by the following equation.

Zn(s) + 2HCl (aq) ⟶ ZnCl₂ (aq) + H₂↑

Answer

The given equation tells us:

1. About the reactants involved and the products formed as a result of the reaction and their state. Zinc (Zn) and Hydrochloric acid (HCl) are the reactants. Zinc is in solid state and HCl is in aqueous solution state. The product Zinc Chloride (ZnCl₂) is also in aqueous solution state and Hydrogen is in gaseous state.
2. About the number of molecules of each substance taking part and formed in the reaction. Here one molecule of Zinc and two molecules of Hydrochloric acid react to give one molecule of Zinc chloride and one molecule of Hydrogen gas.
3. About chemical composition of respective molecules. For example, one molecule of Zinc chloride contains one atom of Zinc and two atoms of Chlorine.
4. About molecular mass; that 65 parts by weight of Zinc reacts with 73 parts by weight of Hydrochloric acid to produce 136 parts by weight of Zinc chloride and 2 parts by weight of Hydrogen.
   $\underset{65}{\text{Zn}}\text{(s)} + \underset{73}{\text{2HCl}}\text{(aq)} \longrightarrow \underset{136}{\text{ZnCl}_2}\text{(aq)} + \underset{2}{\text{H}_2}\uparrow$
5. 65 g of Zinc on treatment with 73 g of HCl, will produce 22.4 litres of hydrogen gas at S.T.P.
6. It also proves the law of conservation of mass. According to the above equation, 138 gram of reactants are producing 138 gram of products.

What is the limitation of the reaction given in question 2?

Answer

The equation of the given reaction does not tell us:

1. the time taken for the completion of the reaction.
2. whether heat is given out or absorbed during the reaction.
3. the respective concentrations of the reactants and the products.
4. the rate at which the reaction proceeds.
5. whether the reaction is completed or it is not completed.

Write chemical equations for the following word equations and balance them.

(a) Carbon + Oxygen ⟶ Carbon dioxide

(b) Nitrogen + Oxygen ⟶ Nitrogen monoxide

(c) Calcium + Nitrogen ⟶ Calcium nitride

(d) Calcium oxide + Carbon dioxide ⟶ Calcium carbonate

(e) Magnesium + Sulphuric acid ⟶ Magnesium sulphate + Hydrogen

(f) Sodium reacts with water to form sodium hydroxide and hydrogen.

Answer

The balanced chemical equations for the word equations are given below:

(a) C + O₂ ⟶ CO₂

(b) N₂ + O₂ ⟶ 2NO

(c) 3Ca + N₂ ⟶ Ca₃N₂

(d) CaO + CO₂ ⟶ CaCO₃

(e) Mg + H₂SO₄ ⟶ MgSO₄ + H₂↑

(f) 2Na + 2H₂O ⟶ 2NaOH + H₂↑

Balance the following equations:

(a) Fe + H₂O ⟶ Fe₃O₄ + H₂

(b) Ca + N₂ ⟶ Ca₃N₂

(c) Zn + KOH ⟶ K₂ZnO₂ + H₂

(d) Fe₂O₃ + CO ⟶ Fe + CO₂

(e) PbO + NH₃ ⟶ Pb + H₂O + N₂

(f) Pb₃O₄ ⟶ PbO + O₂

(g) PbS + O₂ ⟶ PbO + SO₂

(h) S + H₂SO₄ ⟶ SO₂ + H₂O

(i) S + HNO₃ ⟶ H₂SO₄ + NO₂ + H₂O

(j) MnO₂ + HCl ⟶ MnCl₂ + H₂O + Cl₂

(k) C + H₂SO₄ ⟶ CO₂ + H₂O + SO₂

(l) KOH + Cl₂ ⟶ KCl + KClO + H₂O

(m) NO₂ + H₂O ⟶ HNO₂ + HNO₃

(n) Pb₃O₄ + HCl ⟶ PbCl₂ + H₂O + Cl₂

(o) H₂O + Cl₂ ⟶ HCl + O₂

(p) NaHCO₃ ⟶ Na₂CO₃ + H₂O + CO₂

(q) HNO₃ + H₂S ⟶ NO₂ + H₂O + S

(r) P + HNO₃ ⟶ NO₂ + H₂O + H₃PO₄

(s) Zn + HNO₃ ⟶ Zn(NO₃)₂ + H₂O + NO₂

Answer

(a) 3Fe + 4H₂O ⟶ Fe₃O₄ + 4H₂

(b) 3Ca + N₂ ⟶ Ca₃N₂

(c) Zn + 2KOH ⟶ K₂ZnO₂ + H₂

(d) Fe₂O₃ + 3CO ⟶ 2Fe + 3CO₂

(e) 3PbO + 2NH₃ ⟶ 3Pb + 3H₂O + N₂

(f) 2Pb₃O₄ ⟶ 6PbO + O₂

(g) 2PbS + 3O₂ ⟶ 2PbO + 2SO₂

(h) S + 2H₂SO₄ ⟶ 3SO₂ + 2H₂O

(i) S + 6HNO₃ ⟶ H₂SO₄ + 6NO₂ + 2H₂O

(j) MnO₂ + 4HCl ⟶ MnCl₂ + 2H₂O + Cl₂

(k) C + 2H₂SO₄ ⟶ CO₂ + 2H₂O + 2SO₂

(l) 2KOH + Cl₂ ⟶ KCl + KClO + H₂O

(m) 2NO₂ + H₂O ⟶ HNO₂ + HNO₃

(n) Pb₃O₄ + 8HCl ⟶ 3PbCl₂ + 4H₂O + Cl₂

(o) 2H₂O + 2Cl₂ ⟶ 4HCl + O₂

(p) 2NaHCO₃ ⟶ Na₂CO₃ + H₂O + CO₂

(q) 2HNO₃ + H₂S ⟶ 2NO₂ + 2H₂O + S

(r) P + 5HNO₃ ⟶ 5NO₂ + H₂O + H₃PO₄

(s) Zn + 4HNO₃ ⟶ Zn(NO₃)₂ + 2H₂O + 2NO₂

Modern atomic symbols are based on the method proposed by

1. Bohr
2. Dalton
3. Berzelius
4. Alchemist

Answer

Berzelius

Reason — Berzelius suggested that the initial letter of an element written in capitals should represent that particular element. This method suggested by him laid the basis of the IUPAC system of chemical symbols and formulae.

The number of carbon atoms in a hydrogen carbonate radical is

1. one
2. two
3. three
4. four

Answer

one

Reason — Hydrogen carbonate radical is represented as HCO₃^-. Thus, it shows that one carbon atom is present in a hydrogen carbonate radical.

The formula of iron (III) sulphate is

1. Fe₃SO₄
2. Fe(SO₄)₃
3. Fe₂(SO₄)₃
4. FeSO₄

Answer

Fe₂(SO₄)₃

Reason — Chemical formula of iron (III) sulphate:

$\text{Fe}^{3+} \space \text{SO}_4^{2-} \\[1em] \overset{\phantom{22}{3}}{\text{Fe}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{Fe}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{3}}{\text{SO}_4} \\[1em] \text{Fe}_2\text{(SO}_4)_3$

∴ Chemical formula of iron (III) sulphate is Fe₂(SO₄)₃

In water, the hydrogen-to-oxygen mass ratio is

1. 1 : 8
2. 1 : 16
3. 1 : 32
4. 1 : 64

Answer

1 : 8

Reason — Hydrogen-to-oxygen mass ratio in water can be found as below:

Molecualr formula of water = H₂O

∴ Hydrogen-to-oxygen mass ratio in water

   = $\dfrac{2}{16}$ = $\dfrac{1}{8}$

   = 1 : 8

The formula of sodium carbonate is Na₂CO₃ and that of calcium hydrogen carbonate is

1. CaHCO₃
2. Ca(HCO₃)₂
3. Ca₂HCO₃
4. Ca(HCO₃)₃

Answer

Ca(HCO₃)₂

Reason — Chemical formula of calcium hydrogen carbonate:

$\text{Ca}^{2+} \space \text{HCO}_3^{1-} \\[1em] \overset{\phantom{22}{2}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{HCO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{2}}{\text{HCO}_3} \\[1em] \text{Ca}\text{(HCO}_3)_2$

∴ Chemical formula of calcium hydrogen carbonate is Ca(HCO₃)₂

The correct atomic symbols for carbon, calcium, copper and cadmium respectively are:

1. Ca, C, Cu, Cd
2. Ca, C, CO, Cd
3. C, Ca, Cu, Cd
4. Ca, Cl, Co, Cd

Answer

C, Ca, Cu, Cd

Reason — Carbon is denoted by the first letter of its name C, Calcium by the first two letters of its name Ca, Cadmium by first and third letters Cd and Copper is denoted by the first two letters of its latin name Cuprum so Cu.

Hydrargyrum is the Latin name of:

1. Gold
2. Silver
3. Mercury
4. Platinum

Answer

Mercury

Reason — The term "Hydrargyrum" is derived from the Greek words "hydr-" meaning water and "argyros" meaning silver. In Latin, "Hydrargyrum" is used to refer to the element mercury, which is a silvery, liquid metal.

In Zn₃(PO₄)₂, the valency of Zn and Phosphate respectively is:

1. 3 and 2
2. 2 and 6
3. 4 and 6
4. 2 and 3

Answer

2 and 3

Reason — Given the formula Zn₃(PO₄)₂ :

$\text{Zn}_3(\text{PO}_4)_2 = \underset{\phantom{2}{3}}{\text{Zn}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{2}{\text{PO}_4} \Rightarrow \overset{\phantom{3}{2}}{\text{Zn}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{3}{\text{PO}_4} \\[0.5em]$

∴ Valency of Zinc is 2 and Phosphate is 3.

In Ca₃(PO₄)_x, the value of x is:

1. 1
2. 2
3. 3
4. 4

Answer

2

Reason — Chemical formula of calcium phosphate:

$\text{Ca}^{2+} \space \text{PO}_4^{3-} \\[1em] \overset{\phantom{22}{2}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{2}}{\text{PO}_4} \\[1em] \text{Ca}_3\text{(PO}_4)_2$

∴ Chemical formula of calcium phosphate is Ca₃(PO₄)₂ hence x = 2.

In C₂H₄, the valency of carbon is:

1. 2
2. 1
3. 4
4. 3

Answer

4

Reason — Given the formula C₂H₄ :

$\text{C}_2\text{H}_4 = \underset{\phantom{2}{2}}{\text{C}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{4}{\text{H}} \Rightarrow \overset{\phantom{3}{4}}{\text{C}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{H}} \\[0.5em]$

∴ Valency of Carbon is 4.

Which of the following represent the correct chemical formula of a compound :

1. NaSO₄
2. AlSO₄
3. NH₄PO₄
4. Ca₃N₂

Answer

Ca₃N₂

Reason — Calcium (Ca) forms a +2 ion (Ca⁺²) and nitride (N^-3) has a –3 charge.

$\text{Ca}^{2+} \space \text{N}^{3-} \\[1em] \overset{\phantom{22}{2}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \space \overset{3}{\text{N}} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{2}{2}}{\text{N}} \\[1em] \text{Ca}_3\text{N}_2$

It is perfectly balanced. Therefore, Ca₃N₂ is correct.

A chemical equation is always balanced to satisfy the conditions of :

1. Law of constant proportions.

2. Gay Lussac's law

3. Law of conservation of mass

4. Law of mass action.

Answer

Law of conservation of mass

Reason — Law of conservation of mass states that the total mass of the substances on either side of the equation is the same.
In Law of constant proportions, a compound always contains the same elements in a fixed ratio by mass (e.g., water always has hydrogen and oxygen in a fixed ratio). Gay Lussac's law deals with volumes of gases in chemical reactions (gases combine in simple whole-number ratios by volume). Law of mass action relates to the rate of a chemical reaction and how it depends on the concentration of reactants.

One "μ" stands for :

1. One atom of any element
2. 1/12th of hydrogen atom
3. An atom of carbon (C-12)
4. 1/12th the mass of carbon atom (C-12).

Answer

1/12th the mass of carbon atom (C-12)

Reason — Atomic mass is expressed in atomic mass units [a.m.u.] or "μ". Atomic mass unit is defined as 1/12 the mass of carbon atom C-12. (The mass of an atom of carbon-12 isotope was given the atomic mass of 12 units, i.e., 12 amu or simply 12 μ ).

Which of the following statements about a compound is incorrect ?

1. A molecule of a compound has atoms of different elements.
2. A compound cannot be separated into its constituent elements by physical methods of separation.
3. A compound retains the physical properties of its constituent elements.
4. The ratio of atoms of different elements in a compound is fixed.

Answer

A compound retains the physical properties of its constituent elements.

Reason — When elements combine to form a compound, the resulting substance has completely different physical and chemical properties from the elements that make it up.
A compound is formed by the chemical combination of two or more different elements. The elements in a compound are chemically bonded, so you need chemical methods (like electrolysis) to separate them and not physical ones like filtration or evaporation. Compounds always have a definite composition.

Which of the following reactions is not correct according to the laws?

1. P₄ + 5O₂ ⟶ P₄O₁₀

2. C₂H₆ ⟶ CO₂ + H₂O

3. 2Zn + O₂ ⟶ 2ZnO

4. CH₄ + 2O₂ ⟶ CO₂ + 2H₂O

Answer

C₂H₆ ⟶ CO₂ + H₂O

Reason — C₂H₆ ⟶ CO₂ + H₂O
This equation does not follow the Law of conservation of mass because reactant side has 2 carbon atoms and 6 hydrogen atoms whereas product side has only 1 carbon atom and 2 hydrogen atoms. Since both the sides do not have the same number of atoms, it violates the Law of conservation of mass which states that matter can neither be created nor destroyed in a chemical reaction.

Other reactions are balanced. The total number of atoms of each element on the reactants side, is the same as the number of atoms in the products formed, in the equation. So, they follow the Law of conservation of mass.

In the compound magnesium nitride (Mg₃N₂):

P — Mg is trivalent, N is divalent.

Q — Mg is divalent. N is trivalent.

R — Mg is divalent, N is divalent.

Which of the statements given above is/are true?

1. Only P

2. Only Q

3. Only R

4. Both P and Q

Answer

Only Q

Reason — The valency of elements in compound magnesium nitride (Mg₃N₂) is,

$\text{Mg}_3\text{N}_2 = \underset{\phantom{2}{3}}{\text{Mg}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{2}{\text{N}} \Rightarrow \overset{\phantom{3}{2}}{\text{Mg}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{3}{\text{N}} \\[0.5em]$

Hence, Mg is divalent and N is trivalent.

Copper shows variable valency and forms two different compounds with oxygen — Cu₂O and CuO.

P — A is cuprous oxide, B is cupric oxide.

Q — A is cupric oxide, B is cuprous oxide.

R — A is copper (I) oxide, B is copper (II) oxide.

1. Only P

2. Only Q

3. Only R

4. Both P and R

Answer

Both P and R

Reason — Certain elements exhibit more than one valency and they show variable valency.

For example, copper shows valency of 1 and 2 and forms compounds like, cuprous [copper(I)] oxide and cupric [copper(II)] oxide, respectively.

Hence, both P and R are true.

The figure given below shows the molecule of an element, where 'a' denotes the atom with atomic mass 32.

P — Element is tetratomic with molecular mass 128.

Q — Element is octatomic with molecular mass 256.

R — Element is crown-shaped with molecular mass 256.

1. Only P

2. Only Q

3. Only R

4. Both P and R

Answer

Only Q

Reason — Given element 'a' is having atomic mass 32. The figure shows there are eight 'a' atoms forming a octatomic molecule. So when we calculate the molecular mass it will be

32 x 8 = 256

Hence, element 'a' is octatomic molecule with molecular mass 256.

Fill in the blanks:

(a) Dalton used symbol ............... for oxygen and ............... for hydrogen.

(b) Symbol represents ............... atom of an element.

(c) Symbolic expression for a molecule is called ............... .

(d) Sodium chloride has two radicals. Sodium is a ............... radical while chloride is a ............... radical.

(e) Valency of phosphorus in PCl₃ is ............... and in PCl₅ is ............... .

(f) Valency of Iron in FeCl₂ is ............... and in FeCl₃ it is ............... .

(g) Formula of iron (III) carbonate is ............... .

Answer

(a) Dalton used symbol $\bigcirc$ for oxygen and ${\bigcirc}\mathllap{\bullet\phantom{\space}}$ for hydrogen.

(b) Symbol represents one atom of an element.

(c) Symbolic expression for a molecule is called molecular formula .

(d) Sodium chloride has two radicals. Sodium is a basic radical while chloride is a acidic radical.

(e) Valency of phosphorus in PCl₃ is 3 and in PCl₅ is 5 .

(f) Valency of Iron in FeCl₂ is 2 and in FeCl₃ it is 3 .

(g) Formula of iron (III) carbonate is Fe₂(CO₃)₃ .

Name the following compounds and state how many atoms of each kind are present in one molecule of these compounds.

(a) H₂SO₄

(b) HClO₄

(c) K₂Cr₂O₇

(d) KMnO₄

(e) K₄[Fe(CN)₆]

(f) Na₂CrO₄

(g) Mn₃(BO₄)₂

(h) HNO₂

Answer

(a) Atoms in H₂SO₄ are

H ⟶ 2

S ⟶ 1

O ⟶ 4

(b) Atoms in HClO₄

H ⟶ 1

Cl ⟶ 1

O ⟶ 4

(c) Atoms in K₂Cr₂O₇

K ⟶ 2

Cr ⟶ 2

O ⟶ 7

(d) Atoms in KMnO₄

K ⟶ 1

Mn ⟶ 1

O ⟶ 4

(e) Atoms in K₄[Fe(CN)₆]

K ⟶ 4

Fe ⟶ 1

C ⟶ 6

N ⟶ 6

(f) Atoms in Na₂CrO₄

Na ⟶ 2

Cr ⟶ 1

O ⟶ 4

(g) Atoms in Mn₃(BO₄)₂

Mn ⟶ 3

B ⟶ 2

O ⟶ 8

(h) Atoms in HNO₂

H ⟶ 1

N ⟶ 1

O ⟶ 2

Correct the following statements.

(a) A molecular formula represents an element.

(b) Molecular formula of water is H₂O₂.

(c) A molecule of sulphur is monoatomic.

(d) CO and Co both represent cobalt.

(e) Formula of iron (III) oxide is FeO.

Answer

(a) A molecular formula represents the molecule of an element or of a compound.

(b) Molecular formula of water is H₂O.

(c) A molecule of sulphur is octatomic.

(d) CO represents Carbon Monoxide and Co represents cobalt.

(e) Formula of iron (III) oxide is Fe₂O₃.

Give the empirical formula of :

1. Benzene (C₆H₆)
2. Glucose (C₆H₁₂O₆)
3. Acetylene (C₂H₂)
4. Acetic acid (CH₃COOH)

Answer

1. Ratio of C and H atoms in Benzene (C₆H₆) = 6 : 6
   Simplest Ratio = 1 : 1
   ∴ Empirical formula of Benzene (C₆H₆) = CH
2. Ratio of C, H and O atoms in Glucose (C₆H₁₂O₆) = 6 : 12 : 6
   Simplest Ratio = 1 : 2 : 1
   ∴ Empirical formula of Glucose (C₆H₁₂O₆) = CH₂O
3. Ratio of C and H atoms in Acetylene (C₂H₂) = 2 : 2
   Simplest Ratio = 1 : 1
   ∴ Empirical formula of Acetylene (C₂H₂) = CH
4. Ratio of C, H and O atoms in Acetic acid (CH₃COOH) = 2 : 4 : 2
   Simplest Ratio = 1 : 2 : 1
   ∴ Empirical formula of Acetic acid (CH₃COOH) = CH₂O

Complete the following table.

Answer

The completed table is given below:

What is the valency of :

(a) fluorine in CaF₂

(b) sulphur in SF₆

(c) phosphorus in PH₃

(d) carbon in CH₄

(e) Manganese in MnO₂

(f) Copper in Cu₂O

(g) Magnesium in Mg₃N₂

(h) nitrogen in the following compounds :

(i) N₂O₃ (ii) N₂O₅ (iii) NO₂ (iv) NO

Answer

(a) Valency of fluorine in CaF₂ is -1.

(b) Valency of sulphur in SF₆ is +6.

(c) Valency of phosphorus in PH₃ is +3.

(d) Valency of carbon in CH₄ is +4.

(e) Valency of manganese in MnO₂ is +4.

(f) Valency of copper in Cu₂O is +1.

(g) Valency of magnesium in Mg₃N₂ is +2.

(h) Valency of nitrogen in the given compounds :

1. N₂O₃ = +3
2. N₂O₅ = +5
3. NO₂ = +4
4. NO = +2

Why should an equation be balanced ? Explain with the help of a simple equation.

Answer

An equation must be balanced in order to comply with the "Law of Conservation of Matter", which states that matter is neither created nor destroyed in the course of a chemical reaction. An unbalanced equation would imply that atoms have been created or destroyed.

For example, consider the following unbalanced equation:

KNO₃ ⟶ KNO₂ + O₂

In this equation, there are 3 oxygen atoms on the left side, but 4 oxygen atoms on the right side. This means that the equation is not balanced and does not satisfy the law of conservation of mass.

The balanced form of the equation is:

2KNO₃ ⟶ 2KNO₂ + O₂

Now there are 6 oxygen atoms on both the sides. This means that the law of conservation of mass is satisfied and the equation correctly represents the reaction.

Define Atomic mass unit.

Answer

Atomic mass unit is defined as 1⁄12 the mass of an carbon atom C-12.

Define Basic radical.

Answer

A Basic radical is the radical that remains after a base molecule loses a hydroxyl ion (OH^-). Basic radicals typically have a positive charge. They are also called electropositive radicals or cations.

Balance the following equations.

(a) Al₂O₃ + H₂SO₄ ⟶ Al₂(SO₄)₃ + H₂O

(b) NH₃ + Cl₂ ⟶ NH₄Cl + N₂

(c) C₅H₁₂ (pentane) + O₂ ⟶ CO₂ + H₂O

(d) C₄H₁₀ (butane) + O₂ ⟶ CO₂ + H₂O

(e) FeSO₄ + H₂SO₄ + Cl₂ ⟶ Fe₂(SO₄)₃ + HCl

(f) NaCl + MnO₂ + H₂SO₄ ⟶ NaHSO₄+ MnSO₄ + H₂O + Cl₂

(g) Na₂Cr₂O₇ + HCl ⟶ NaCl + CrCl₃ + H₂O + Cl₂

(h) KMnO₄ + H₂SO₄ + H₂S ⟶ K₂SO₄ + MnSO₄ + H₂O + S

Answer

(a) Al₂O₃ + 3H₂SO₄ ⟶ Al₂(SO₄)₃ + 3H₂O

(b) 8NH₃ + 3Cl₂ ⟶ 6NH₄Cl + N₂

(c) C₅H₁₂ (pentane) + 8O₂ ⟶ 5CO₂ + 6H₂O

(d) 2C₄H₁₀ (butane) + 13O₂ ⟶ 8CO₂ + 10H₂O

(e) 2FeSO₄ + H₂SO₄ + Cl₂ ⟶ Fe₂(SO₄)₃ + 2HCl

(f) 2NaCl + MnO₂ + 3H₂SO₄ ⟶ 2NaHSO₄ + MnSO₄ + 2H₂O + Cl₂

(g) Na₂Cr₂O₇ + 14HCl ⟶ 2NaCl + 2CrCl₃ + 7H₂O + 3Cl₂

(h) 2KMnO₄ + 3H₂SO₄ + 5H₂S ⟶ K₂SO₄ + 2MnSO₄ + 8H₂O + 5S

Atomic mass of calcium is 40 μ. What does it signify ?

Answer

The atomic mass of calcium being 40 amu signifies that, on average, a calcium atom weighs 40 times the mass of one twelfth of a carbon-12 atom.

Write the balanced chemical equations of the following word equations.

(a) Sodium hydroxide + sulphuric acid ⟶ sodium sulphate + water

(b) Potassium bicarbonate + sulphuric acid ⟶ potassium sulphate + carbon dioxide + water

(c) Iron + sulphuric acid ⟶ ferrous sulphate + hydrogen

(d) Chlorine + sulphur dioxide + water ⟶ sulphuric acid + hydrogen chloride

(e) Silver nitrate ⟶ silver + nitrogen dioxide + oxygen

(f) Copper + nitric acid ⟶ copper nitrate + nitric oxide + water

(g) Ammonia + oxygen ⟶ nitric oxide + water

(h) Barium chloride + sulphuric acid ⟶ barium sulphate + hydrochloric acid

(i) Zinc sulphide + oxygen ⟶ zinc oxide + sulphur dioxide

(j) Aluminium carbide + water ⟶ aluminium hydroxide + methane

(k) Iron pyrites (FeS₂) + oxygen ⟶ ferric oxide + sulphur dioxide

(l) Potassium permanganate + hydrochloric acid ⟶ potassium chloride + manganese chloride + chlorine + water

(m) Aluminium sulphate + sodium hydroxide ⟶ sodium sulphate + sodium meta aluminate + water

(n) Aluminium + sodium hydroxide + water ⟶ sodium meta aluminate + hydrogen

(o) Potassium dichromate + sulphuric acid ⟶ potassium sulphate + chromium sulphate + water + oxygen

(p) Potassium dichromate + hydrochloric acid ⟶ potassium chloride + chromium chloride + water + chlorine

(q) Sulphur + nitric acid ⟶ sulphuric acid + nitrogen dioxide + water

(r) Sodium chloride + manganese dioxide + sulphuric acid ⟶ sodium hydrogen sulphate + manganese sulphate + water + chlorine

Answer

(a) 2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O

(b) 2KHCO₃ + H₂SO₄ ⟶ K₂SO₄ + 2CO₂ + 2H₂O

(c) Fe + H₂SO₄ ⟶ FeSO₄ + H₂

(d) Cl₂ + SO₂ + 2H₂O ⟶ H₂SO₄ + 2HCl

(e) 2AgNO₃ ⟶ 2Ag + 2NO₂ + O₂

(f) 3Cu + 8HNO₃ ⟶ 3Cu(NO₃)₂ + 2NO + 4H₂O

(g) 4NH₃ + 5O₂ ⟶ 4NO + 6H₂O

(h) BaCl₂ + H₂SO₄ ⟶ BaSO₄ + 2HCl

(i) 2ZnS + 3O₂ ⟶ 2ZnO + 2SO₂

(j) Al₄C₃ + 12H₂O ⟶ 4Al(OH)₃ + 3CH₄

(k) 4FeS₂ + 11O₂ ⟶ 2Fe₂O₃ + 8SO₂

(l) 2KMnO₄ + 16HCl ⟶ 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O

(m) Al₂(SO₄)₃ + 8NaOH ⟶ 3Na₂SO₄ + 2NaAlO₂ + 4H₂O

(n) 2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

(o) 2K₂Cr₂O₇ + 8H₂SO₄ ⟶ 2K₂SO₄ + 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

(p) K₂Cr₂O₇ + 14HCl ⟶ 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

(q) S + 6HNO₃ ⟶ H₂SO₄ + 6NO₂ + 2H₂O

(r) 2NaCl + MnO₂ + 3H₂SO₄ ⟶ 2NaHSO₄ + MnSO₄ + 2H₂O + Cl₂

Sodium chloride reacts with silver nitrate to produce silver chloride and sodium nitrate

(a) Write the equation.

(b) Check whether it is balanced, if not balance it.

(c) Find the weights of reactants and products.

(d) State the law that this equation satisfies.

Answer

(a) NaCl + AgNO₃ ⟶ AgCl + NaNO₃

(b) The equation is balanced.

(c) Calculating the weights of reactants and products from the equation:

$\begin{matrix} \text{NaCl} & + & \text{AgNO}_3 & \longrightarrow & \text{AgCl} & + & \text{NaNO}_3 \\ 23 + 35.5 & + & 108 + 14 + 3(16) & & 108 + 35.5 & + & 23 + 14 + 3(16) \\ \Rightarrow 58.5 & + & 170 & & 143.5 & + & 85 \\ \Rightarrow & 228.5 & & & & 228.5 & \end{matrix}$

Thus, weights of reactants = weights of products = 228.5 g

(d) The equation satisfies the law of conservation of mass. It states that the total mass of the substances on either side of the equation is the same.

What information does the following chemical equations convey ?

(a) Zn + H₂SO₄ ⟶ ZnSO₄ + H₂

(b) Mg + 2HCl ⟶ MgCl₂ + H₂

Answer

$\text{(a) }\underset{65}{\text{Zn}} + \underset{98}{\text{H}_2\text{SO}_4} \longrightarrow \underset{161}{\text{Zn}\text{SO}_4} + \underset{2}{\text{H}_2} \\[1em]$

The above balanced equation conveys the following information:

1. One molecule of zinc reacts with one molecule of sulphuric acid to produce one molecule of zinc Sulphate and one molecule of hydrogen.
2. 65 g of zinc reacts with 98 g of sulphuric acid to produce 161 g of zinc Sulphate and 2 g of hydrogen.
3. 65 g of zinc reacts with 98 g of sulphuric acid to produce 22.4 litres of hydrogen gas at S.T.P.
4. It also proves the law of conservation of mass. According to the above equation, 163 gram of reactants are producing 163 gram of products.

$\text{(b) }\underset{24}{\text{Mg}} + \underset{73}{\text{2HCl}} \longrightarrow \underset{95}{\text{MgCl}_2} + \underset{2}{\text{H}_2} \\[1em]$

The above balanced equation conveys the following information:

1. One molecule of magnesium reacts with two molecules of hydrochloric acid to produce one molecule of magnesium chloride and one molecule of hydrogen.
2. 24 g of magnesium reacts with 73 g of hydrochloric acid to produce 95 g of magnesium chloride and 2 g of hydrogen.
3. 24 g of magnesium reacts with 73 g of hydrochloric acid to produce 22.4 litres of hydrogen gas at S.T.P.
4. It also proves the law of conservation of mass. According to the above equation, 97 gram of reactants are producing 97 gram of products.

(a) What are poly-atomic ions ? Give two examples.

(b) Name and state the fundamental law that every equation must fulfill.

Answer

(a) Polyatomic ions are ions composed of two or more atoms that are covalently bonded together and carry a net electrical charge. Examples — Nitrate ion (NO₃^-) and Sulphate ion (SO₄^2-).

(b) Every equation must fulfill the "Law of Conservation of Matter". It states that matter is neither created nor destroyed in the course of a chemical reaction. Thus, the total mass of the substances on either side of the equation must be the same.

Give the information conveyed by the chemical formula of a compound.

Answer

By looking at the chemical formula, we understand the ratio in which different atoms are united to form the molecule.

Write the significance of a molecular formula.

Answer

The molecular formula of a compound has quantitative significance. It represents:

1. both the molecule and the molecular mass of the compound.
2. the respective numbers of different atoms present in one molecule of the compound.
3. the ratios of the respective masses of the elements present in the compound.

For example, the formula CO₂ means that:

1. the molecular formula of carbon dioxide is CO₂
2. each molecule contains one carbon atom joined by chemical bonds with two oxygen atoms;
3. the molecular mass of carbon dioxide is 44, given that atomic mass of carbon is 12 and that of oxygen is 16.

What do you understand by radicals : What are basic and acidic radicals ? Explain with examples.

Answer

A radical is an atom or a group of atoms of the same or of different elements that behaves as a single unit with a positive or a negative charge.

An Acid radical is the radical that remains after an acidic molecule loses a hydrogen ion (H⁺). Acid radicals typically have a negative charge. They are also called electronegative radicals or anions.

A Basic radical is the radical that remains after a base molecule loses a hydroxyl ion (OH^-). Basic radicals typically have a positive charge. They are also called electropositive radicals or cations.

For example, in the compound ammonium carbonate (NH₄)₂CO₃, ammonium (NH₄⁺) is a basic radical with combining power 1 and carbonate (CO₃^2-) is an acidic radical with combining power 2.

Give four examples of compounds with variable valency.

Answer

(a) Account for the existence of radicals in the atmosphere.

(b) How are they produced in the atmosphere?

(c) What is the role of radicals in the atmosphere?

Answer

(a) The major radicals present in the atmosphere are:

(i) Hydroxyl radical (OH●): It is also called as detergent of the atmosphere because it helps in removing air pollutants like methane, carbon monoxide and other harmful gases.

(ii) Nitrate radical (NO₃●): It converts harmful gases into forms that can be washed out by rain.

(iii) Chlorine radical (Cl●): It is released from chlorofluorocarbons and reacts to destroy ozone layer.

(b) Atmospheric radicals are formed when stable molecules absorb energy, mainly from sunlight, and their chemical bonds break. This process produces highly reactive species with unpaired electron.

(c) Atmospheric radicals play a crucial role in both cleaning the atmosphere and influencing important environmental processes.

Free radicals are necessary for some biological functions, but excessive radicals cause significant damage. State the ways in which they can cause damage.

Answer

Excessive radicals cause following damages :

1. Excess free radicals can damage cell membranes.
2. They can harm proteins and enzymes.
3. They can affect DNA and genetic material.
4. They speed up ageing processes.

It is necessary to maintain a healthy balance of atmospheric radicals. Evaluate this statement.

Answer

A healthy balance of atmospheric radicals is necessary because some radicals help in cleaning the atmosphere, while others can harm it.

Hydroxyl radicals (OH•) and nitrate radicals (NO₃•) remove pollutants such as methane, carbon monoxide and other harmful gases from the air. However, excess chlorine radicals (Cl•) released from chlorofluorocarbons can destroy the ozone layer.

Hence, atmospheric radicals must be maintained in balance to keep the atmosphere clean without causing environmental damage.

Elements X, Y and Z have 3, 7 and 6 electrons in their valence shells respectively. Write the formula of the compound formed between :

(a) X and Y

(b) X and Z

Answer

(a) Valency of X is +3 and Y is -1. Formula of the compound formed between X and Y is :

$\text{X}^{3+} \space \text{Y}^{1-} \\[1em] \overset{\phantom{22}{3}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{Y}} \Rightarrow \underset{\phantom{2}{1}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \underset{{3}}{\text{Y}} \\[1em] \text{X}\text{Y}_3$

∴ Chemical formula of the compound formed between X and Y is XY₃

(b) Valency of X is +3 and Z is -2. Formula of the compound formed between X and Z is :

$\text{X}^{3+} \space \text{Z}^{2-} \\[1em] \overset{\phantom{22}{3}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{Z}} \Rightarrow \underset{\phantom{2}{2}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \underset{{3}}{\text{Z}} \\[1em] \text{X}_2\text{Z}_3$

∴ Chemical formula of the compound formed between X and Z is X₂Z₃

The following figure represents the structural formula of a chemical compound.

Answer the questions given below:

(a) How many Carbon and Hydrogen atoms are present in the compound.

(b) Write the molecular and empirical formulae of the compound.

(c) Calculate the percentage composition of all the elements present in the compound. [At. wt. of C = 12, H = 1]

Answer

(a) 6 Carbon and 12 Hydrogen atoms are present.

(b) Its molecular formula is C₆H₁₂.
As the simplest ration between C and H is 1:2, hence the empirical formula is CH₂

(c) Relative molecular mass of C₆H₁₂
   = (6 x 12) + (1 x 12)
   = 72 + 12
   = 84 g

84 g of C₆H₁₂ contains 72 g of Carbon

∴ 100 g of C₆H₁₂ contains $\dfrac{72 \times 100}{84}$ g of Carbon

= $\dfrac{7200}{84}$ = 85.7 g of Carbon.

84 g of C₆H₁₂ contains 12 g of Hydrogen

∴ 100 g of C₆H₁₂ contains $\dfrac{12 \times 100}{84}$ g of Hydrogen

= $\dfrac{1200}{84}$ = 14.3 of Hydrogen.

∴ In C₆H₁₂, C = 85.7% and H = 14.3%

Calculate the molecular mass of the following :

(i) Na₂SO₄.10H₂O

(ii) (NH₄)₂CO₃

(iii) (NH₂)₂CO

(iv) Mg₃N₂

Given atomic mass of Na = 23, H = 1, O = 16, C = 12, N = 14, Mg = 24, S = 32

Answer

(i) Molecular mass of Na₂SO₄.10H₂O
     = 2 x 23 + 32 + 4 x 16 + 10(2 x 1 + 16)
     = 46 + 32 + 64 + 180
     = 322 amu

(ii) Molecular mass of (NH₄)₂CO₃
     = 2(14 + 4 x 1) + 12 + 3 x 16
     = 36 + 12 + 48
     = 96 amu

(iii) Molecular mass of (NH₂)₂CO
     = 2(14 + 2 x 1) + 12 + 16
     = 32 + 12 + 16
     = 60 amu

(iv) Molecular mass of Mg₃N₂
     = 3 x 24 + 2 x 14
     = 72 + 28
     = 100 amu

Calculate the relative molecular masses of

(a) CHCl₃

(b) (NH₄)₂Cr₂O₇

(c) CuSO₄.5H₂O

(d) (NH₄)₂SO₄

(e) CH₃COONa

(f) Potassium chlorate, KClO₃

(g) Ammonium chloroplatinate, (NH₄)₂PtCl₆

[At. mass : C = 12, H = 1, O = 16, Cl = 35.5, N = 14, Cu = 63.5, S = 32, Na = 23, K = 39, Pt = 195, Ca = 40, P = 31, Mg = 24, Cr = 52]

Answer

(a) The relative molecular mass of CHCl₃
     = 12 + 1 + 3 x 35.5
     = 12 + 1 + 106.5
     = 119.5 amu

(b) The relative molecular mass of (NH₄)₂Cr₂O₇
     = 2(14 + 4 x 1) + 2 x 52 + 7 x 16
     = 36 + 104 + 112
     = 252 amu

(c) The relative molecular mass of CuSO₄.5H₂O
     = 63.5 + 32 + 4 x 16 + 5(2 x 1 + 16)
     = 63.5 + 32 + 64 + 90
     = 249.5 amu

(d) The relative molecular mass of (NH₄)₂SO₄
     = 2(14 + 4 x 1) + 32 + 4 x 16
     = 36 + 32 + 64
     = 132 amu

(e) The relative molecular mass of CH₃COONa
     = 12 + 3 + 12 + 16 + 16 + 23
     = 82 amu

(f) The relative molecular mass of KClO₃
     = 39 + 35.5 + 3 x 16
     = 39 + 35.5 + 48
     = 122.5 amu

(g) The relative molecular mass of (NH₄)₂PtCl₆
     = 2(14 + 4 x 1) + 195 + 6 x 35.5
     = 36 + 195 + 213
     = 444 amu

Find the percentage mass of water in Epsom salt, MgSO₄.7H₂O.

Answer

Relative molecular mass of MgSO₄.7H₂O
   = 24 + 32 + 4 x 16 + 7(2 x 1 + 16)
   = 24 + 32 + 64 + 126
   = 246 amu

246 g of Epsom salt contains 126 g of water of crystallisation

∴ 100 g of Epsom salt contains $\dfrac{126 \times 100}{246}$

= $\dfrac{12600}{246}$ = 51.22 g of water of crystallisation.

∴ % mass of water in Epsom salt, MgSO₄.7H₂O = 51.22%

Calculate the percentage of phosphorus in:

(a) Calcium hydrogen phosphate, Ca(H₂PO₄)₂

(b) Calcium phosphate, Ca₃(PO₄)₂

Answer

(a) Relative molecular mass of Ca(H₂PO₄)₂
     = 40 + 2(2 x 1 + 31 + 4 x 16)
     = 40 + 2(2 + 31 + 64)
     = 40 + 194
     = 234 amu

Wt. of P in Ca(H₂PO₄)₂ = 2 x 31 = 62 g

% of P = $\dfrac{\text{Wt. of P}}{\text{Total Wt. of Ca(H}_2\text{PO}_4)_2}\times 100$

= $\dfrac{62}{234}$ x 100 = 26.5%

∴ Phosphorus in Calcium hydrogen phosphate is 26.5%

(b) Relative molecular mass of Calcium phosphate, Ca₃(PO₄)₂
     = 3 x 40 + 2(31 + 4 x 16)
     = 120 + 2(31 + 64)
     = 120 + 190
     = 310 amu

Wt. of P in Ca₃(PO₄)₂ = 2 x 31 = 62 g

% of P = $\dfrac{\text{Wt. of P}}{\text{Total Wt. of Ca}_3\text{(PO}_4)_2}\times 100$

= $\dfrac{62}{310}$ x 100 = 20%

∴ Phosphorus in Calcium phosphate is 20%

Calculate the percentage composition of each element in Potassium chlorate, KClO₃.

Answer

Relative molecular mass of KClO₃
   = 39 + 35.5 + 3 x 16
   = 39 + 35.5 + 48
   = 122.5 amu

122.5 g of KClO₃ contains 39 g of Potassium

∴ 100 g of KClO₃ contains $\dfrac{39 \times 100}{122.5}$ g of Potassium

= $\dfrac{39000}{1225}$ = 31.83 g of Potassium

122.5 g of KClO₃ contains 35.5 g of Chlorine

∴ 100 g of KClO₃ contains $\dfrac{35.5 \times 100}{122.5}$ g of Chlorine

= $\dfrac{35500}{1225}$ = 28.98 g of Chlorine

122.5 g of KClO₃ contains 48 g of Oxygen

∴ 100 g of KClO₃ contains $\dfrac{48 \times 100}{122.5}$ g of Oxygen

= $\dfrac{48000}{1225}$ = 39.18 g of Oxygen

∴ In KClO₃ : K = 31.83%, Cl = 28.98% and O = 39.18%

Urea is a very important nitrogenous fertilizer. Its formula is CON₂H₄. Calculate the percentage of carbon in urea.

(C= 12, O = 16, N = 14 and H = 1)

Answer

Relative molecular mass of CON₂H₄
   = 12 + 16 + 2 x 14 + 4 x 1
   = 12 + 16 + 28 + 4
   = 60 amu

Wt. of C in CON₂H₄ = 12 g

% of C = $\dfrac{\text{Wt. of C}}{\text{Total Wt. of CON}_2\text{H}_4}\times 100$

= $\dfrac{12}{60}$ x 100 = 20%

∴ Carbon in Urea is 20%

Calculate the percentage of iron in Ferric oxide Fe₂O₃.

Answer

Relative molecular mass of Ferric oxide (Fe₂O₃)
  = 56 x 2 + 16 x 3
  = 112 + 48
  = 160 amu

Since 160 g of Ferric oxide (Fe₂O₃ ) contains 112 g of iron

∴ 100 g of contains Ferric oxide (Fe₂O₃ ) contains $\dfrac{112}{160}$ x 100 = 70%

∴ Percentage of Iron in Ferric oxide (Fe₂O₃ ) is 70%

Calculate amount of Nitrogen in one bag (50 kg) of Urea.

Answer

Molecular formula of Urea is CON₂H₄

Relative molecular mass of CON₂H₄
   = 12 + 16 + 2 x 14 + 4 x 1
   = 12 + 16 + 28 + 4
   = 60 amu

Wt. of N in CON₂H₄ = 28 g

% of N = $\dfrac{\text{Wt. of N}}{\text{Total Wt. of CON}_2\text{H}_4}\times 100$

= $\dfrac{28}{60}$ x 100 = 46.67%

Wt. of N in 50 kg CON₂H₄

= $\dfrac{46.67}{100}$ x 50 = 23.33 kg

∴ Amount of Nitrogen in one bag (50 kg) of Urea is 23.33 kg.

Calculate percentage of platinum in [(NH₄)₂ PtCl₆]. Ammonium chloride platinate in nearest whole number.

Answer

The relative molecular mass of (NH₄)₂ PtCl₆
     = 2(14 + 4 x 1) + 195 + 6 x 35.5
     = 36 + 195 + 213
     = 444 amu

Wt. of Pt in [(NH₄)₂ PtCl₆] = 195 g

∴ Percentage of platinum in (NH₄)₂PtCl₆ is

    = $\dfrac{195}{444}$ x 100

    = 43.91 %

∴ Rounding to the nearest whole number, the percentage of platinum in ammonium chloride platinate (NH₄)₂ PtCl₆ is 44%.

Calculate percentage composition of the elements in calcium phosphate Ca₃(PO₄)₂.

Answer

Relative molecular mass of Calcium phosphate, Ca₃(PO₄)₂
     = 3 x 40 + 2(31 + 4 x 16)
     = 120 + 2(31 + 64)
     = 120 + 190
     = 310 amu

310 g of Ca₃(PO₄)₂ contains 120 g of Calcium

∴ 100 g of Ca₃(PO₄)₂ contains $\dfrac{120 \times 100}{310}$ g of Calcium

= $\dfrac{12000}{310}$ = 38.70 % of Calcium.

310 g of Ca₃(PO₄)₂ contains 62 g of Phosphorus

∴ 100 g of Ca₃(PO₄)₂ contains $\dfrac{62 \times 100}{310}$ g of Phosphorus

= $\dfrac{6200}{310}$ = 20 % of Phosphorus.

310 g of Ca₃(PO₄)₂ contains 128 g of Oxygen.

∴ 100 g of Ca₃(PO₄)₂ contains $\dfrac{128 \times 100}{310}$ g of Oxygen.

= $\dfrac{12800}{310}$ = 41.3 % of Oxygen.

∴ In Ca₃(PO₄)₂ : Ca = 38.70%, P = 20% and O = 41.3%

Assertion (A): The atomic mass of sodium is 23 amu.

Reason (R): An atom of sodium is 23 times heavier than an atom of carbon with mass 12 amu.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — The atomic mass of sodium is 23 amu. Hence, the assertion (A) is true.

An atom of sodium is 23 times heavier than one-twelfth the mass of a carbon-12 atom, not 23 times heavier than an atom of carbon with mass 12 amu. Hence, reason (R) is false.

Therefore, A is true but R is false is the correct option.

Assertion (A): An atom is the smallest part of matter which can take part in a chemical reaction.

Reason (R): Atoms of every element can exist independently.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — An atom is the smallest particle of an element and can take part in a chemical reaction. Hence, the assertion (A) is true.

Atoms may or may not exist independently.
For example:

Noble gas atoms like He, Ne can exist freely.

But atoms like H, O, and N do not exist alone under normal conditions; they exist as diatomic molecules (H₂, O₂, N₂). Hence, reason (R) is false.

Therefore, A is true but R is false is the correct option.

Assertion (A): All equations need to be balanced.

Reason (R): An unbalanced equation would imply that atoms have been created or destroyed.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — An equation must be balanced in order to comply with the "Law of Conservation of Matter", which states that matter is neither created nor destroyed in the course of a chemical reaction. Hence, the assertion (A) is true.
An unbalanced equation would imply that atoms have been created or destroyed. So, R us true.

Hence, both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Assertion (A): PCl₃, is known as phosphorus trichloride while AlCl₃, is aluminium chloride and not aluminium trichloride.

Reason (R) : Phosphorus shows variable valency. Aluminium does not show variable valency

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as phosphorus can show variable valency (like 3 and 5), so we name its compounds with prefixes like phosphorus trichloride (PCl₃).
R is true as aluminium, however, shows a fixed valency of 3, so AlCl₃ is simply called aluminium chloride without needing a prefix.

Hence, both assertion (A) and reason (R) are true, and reason (R) correctly explains assertion (A).

Assertion (A): In a chemical reaction, the total mass of the products remains the same as that of the reactants.

Reason (R): A chemical reaction involves a simple exchange of partners and no new species are formed.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — In a chemical reaction, the total mass of the substances on either side of the equation is the same, this is referred to be as law of conservation of mass. Hence, the assertion (A) is true.
In a chemical reaction the products are formed by the rearrangement of atoms or elements in the reactants. So, R is true.

Hence, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion (A): Magnesium phosphate is written as Mg₃(PO₄)₂.

Reason (R): The valencies of magnesium and phosphate are three and two respectively.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — The molecular formula of Magnesium phosphate is Mg₃(PO₄)₂. Hence, the given assertion (A) is true.

Given the formula Mg₃(PO₄)₂ :

$\text{Mg}_3(\text{PO}_4)_2 = \underset{\phantom{2}{3}}{\text{Mg}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{2}{\text{PO}_4} \Rightarrow \overset{\phantom{3}{2}}{\text{Mg}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{3}{\text{PO}_4} \\[0.5em]$

∴ Valency of magnesium (Mg) is 2 and Phosphate is 3. Hence, the reason (R) is false.

Therefore, A is true but R is false is the correct option.

Assertion (A): A chemical equation is written to represent molecules.

Reason (R): This is necessary for the law of conservation of mass

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — A chemical equation represents molecules and it is necessary for the chemical equation to follow the law of conservation of mass. Hence, both assertion (A) and reason (R) are true.
However, balancing the equation is what ensures the law of conservation of mass. Therefore, reason (R) is not the correct explanation of assertion (A).

Therefore, Both A and R are true but R is not the correct explanation of A is not the correct option.

Assertion (A): Chemical combination always takes place between two elements.

Reason (R): A single substance is formed in a chemical combination.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — Chemical combination may take place between two or more elements. Hence, the assertion (A) is false.
A compound is a substance formed by the chemical combination of two or more elements in a fixed proportion. So, a single substance is formed in a chemical combination. Hence, the reason (R) is true.

Thereofore, A is false but R is true is the correct option.

Assertion (A): Hydroxyl radicals (OH•) are considered the "detergents" of the atmosphere.

Reason (R): Hydroxyl radicals are highly reactive and oxidise many pollutants, breaking them down into less harmful substances.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as hydroxyl radicals (OH•) are called the “detergents” of the atmosphere. This is true because they help clean the air.
R is true as (OH•) radicals are highly reactive and oxidise many pollutants like methane and carbon monoxide, breaking them down into less harmful substances that can be removed from the atmosphere.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion (A): The concentration of free radicals in the atmosphere is high during the night.

Reason (R): Free radicals are produced by the photolysis of molecules by solar radiation.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — Free radicals in the atmosphere are mainly formed when stable molecules absorb energy from sunlight and undergo photolysis. Hence, the R is true.

However, the statement that the concentration of free radicals is high during the night is not generally correct because photolysis depends on solar radiation, which is absent at night. Hence, the A is false.

Therefore, A is false but R is true is the correct option.

Atoms being extremely small, cannot be seen or weighed directly. But indirect methods of physics have enabled us to know the absolute mass of nearly all kinds of atoms. The mass of a hydrogen atom is found to be 1.66 × 10^-24 g while that of a carbon atom is 1.9926 × 10^-23 g. As these masses are too small, it is not convenient to use kilograms or grams as unit. It has, therefore, been considered appropriate to use the mass of some standard atom as a unit and then relate masses of other atoms to it.

(a) What do you understand by the statement "R.A.M. of silver is 108" ?

(b) Which element is considered as the standard for atomic masses ?

(c) What is the difference between R.A.M. and R.M.M.? Give an example to explain.

(d) What is the significance of amu or μ ?

(e) How is 1 amu related to grams ?

(f) Is the atomic mass expressed in amu the actual mass of an atom of that element ?

(g) Why is amu preferred instead of grams or kilogram for calculating the atomic mass of an atom ?

Answer

(a) “R.A.M. of silver is 108” means that one atom of silver is 108 times as heavy as 1⁄12^th the mass of one carbon-12 atom.

(b) The element considered as the standard for atomic masses is carbon-12.

(c) R.A.M. means Relative Atomic Mass. It tells us how many times one atom of an element is heavier than $\frac{1}{12}$^th the mass of one carbon-12 atom.

Example:
R.A.M. of oxygen = 16.
This means one atom of oxygen is 16 times heavier than $\frac{1}{12}$^th the mass of one carbon-12 atom.

R.M.M. means Relative Molecular Mass. It tells us how many times one molecule of a substance is heavier than $\frac{1}{12}$^th the mass of one carbon-12 atom.

Example:
R.M.M. of water, H₂O:

R.M.M. of H₂O = 2 × R.A.M. of H + 1 × R.A.M. of O
= 2 × 1 + 16 = 18

So, the R.M.M. of water is 18. This means one molecule of water is 18 times heavier than $\frac{1}{12}$^th the mass of one carbon-12 atom.

Thus, R.A.M. is used for atoms of elements, while R.M.M. is used for molecules of elements or compounds.

(d) The significance of amu is that it provides a convenient unit for expressing the extremely small masses of atoms and molecules. It simplifies calculations by avoiding extremely small values in grams.

(e) 1 amu = 1.66 × 10^-24 g

(f) No, the atomic mass expressed in amu is not the actual mass of an atom in grams. It is a relative mass expressed by comparing the mass of the atom with $\frac{1}{12}$^th the mass of a carbon-12 atom.

(g) The amu is preferred because the actual masses of atoms in grams or kilograms are extremely small and inconvenient to use. The amu gives a simple, practical and manageable scale for comparing atomic masses.

An element X forms a chloride with the formula XCl₃, and an oxide with the formula X₂O₃. This oxide dissolves in water to form a solution that turns blue litmus red.

(a) Identify the element.

(b) Is X a metal or a non-metal ? Justify your answer.

(c) Write the chemical formula for the sulphate of element X.

Answer

(a) The element is phosphorus.

Reason — In XCl₃, chlorine has valency 1. Since one atom of X combines with three chlorine atoms, the valency of X is 3.
But its oxide turns blue litmus red which means it's a non-metal. Phosphorus is a non-metal with valency 3. Formula for phorphorus oxide is P₂O₃ . Boron and aluminium also have valency 3 but boron is a metalloid and aluminium is a metal.

(b) X is a non-metal because its oxide forms an acidic solution in water (turns blue litmus red), which is a characteristic property of non-metals.

(c) The sulphate of phosphorus is P₂(SO₄)₃.
Note — Although the formula P₂(SO₄)₃ can be written using valency rules, this compound does not actually exist in reality. This is because sulphates are formed by metals (which give cations), whereas phosphorus is a non-metal and does not form P³⁺ cations under normal conditions. Hence, P₂(SO₄)₃ is only a theoretical formula based on valency.

A compound is found to have a mass ratio of element A to element B as 3 : 8. If the formula of the compound is AB, what is the relationship between the atomic masses of A and B.

Answer

Atomic mass of A = M_A

Atomic mass of B = M_B

Since the formula of the compound is AB, one atom of A combines with one atom of B.

∴ M_A : M_B = 3 : 8

or,

$\dfrac{\text M_\text A}{\text M_\text B} = \dfrac{3}{8}$

On cross-multiplication,

8M_A = 3M_B

Thus, the relationship between the atomic masses of A and B is

8M_A = 3M_B