The gases hydrogen, oxygen, carbon dioxide, sulphur dioxide and chlorine are arranged in order of their increasing relative molecular masses. Given 8 g of each gas at S.T.P., which gas will contain the least number of molecules and which gas the most?

According to Avogadro's law:

Equal volumes of all gases, under similar conditions of temperature and pressure, contain equal numbers of molecules.

So, 1 mole of each gas contains = 6.02 × 10²³ molecules

Mol. Mass of :

H₂ = 2 g,

O₂ = 32 g,

CO₂ = 12 + 2(16) = 44 g,

SO₂ = 32 + 2(16) = 64 g,

Cl₂ = 2(35.5) = 71 g

(i) 2 g of hydrogen contains molecules = 6.02 × 10²³

So, 8 g of hydrogen contains molecules

= $\dfrac{6.02 × 10^{23}}{2}$ x 8

= 4 × 6.02 × 10²³molecules

= 24.08 × 10²³molecules

(ii) 32 g of oxygen contains molecules = 6.02 × 10²³

So, 8 g of oxygen contains molecules

= $\dfrac{6.02 × 10^{23}}{32}$ x 8

= 1.505 × 10²³molecules

(iii) 44 g of carbon dioxide contains molecules = 6.02 × 10²³

So, 8 g of carbon dioxide contains

= $\dfrac{6.02 × 10^{23}}{44}$ x 8

= 1.09 × 10²³molecules

(iv) 64 g of sulphur dioxide contains molecules = 6.02 × 10²³

So, 8g of sulphur dioxide contains

= $\dfrac{6.02 × 10^{23}}{64}$ x 8

= 0.75 × 10²³molecules

(v) 71 g of chlorine contains molecules = 6.02 × 10²³

So, 8g of chlorine contains

= $\dfrac{6.02 × 10^{23}}{71}$ x 8

= 0.67 × 10²³molecules

Thus Cl₂ < SO₂ < CO₂ < O₂ < H₂

(i) Least number of molecules in Cl₂

(ii) Most number of molecules in H₂