(i) Define pH. If the hydrogen ion concentration of a solution is 10^-12 moles per litre, calculate its pH value.

(ii) What fluid is injected when yellow wasps or white ants bite? How can the pain be relieved?

(iii) A solution of Iron (III) chloride has a pH less than 7. Is the solution acidic or alkaline. Write the equation of its hydrolysis.

(iv) If iron is reacted with dilute sulphuric acid, what will be the product formed ?

(i) pH is defined as the negative logarithm [to the base 10] of the hydrogen ion concentration expressed in moles/litre.

Thus, pH = -log₁₀H⁺. It represents the strength of acids and alkalis, expressed in terms of hydrogen ion concentration [H⁺ aq.]

pH = -log₁₀H⁺ = -log₁₀[10^-12] = 12

Hence, pH value = 12

(ii) When yellow wasps or white ants bite, they inject formic acid into the skin. So by applying base it neutralises the acid and gives relief.

(iii) Solution will be acidic as pH is less than 7.

Hydrolysis of Iron (III) chloride:

FeCl₃ ⟶ Fe³⁺ + 3Cl^-

H₂O ⟶ H⁺ + OH^-

Further, the ferric ions partially combine with hydroxide ions to form ferric hydroxide.

Fe³⁺ + 3OH^- ⟶ Fe(OH)₃

This precipitation removes the hydroxyl ions [OH^-] from the solution leaving a relative excess of hydrogen ions and this excess of hydrogen ions makes the solution acidic.

(iv) Iron sulphate will be formed

Fe + H₂SO₄ ⟶ FeSO₄ + H₂ ↑