Question

1 g of an impure sample of magnesium carbonate (containing no thermally decomposable impurities) on complete thermal decomposition gave 0.44 g of carbon dioxide gas. The percentage of impurity in the sample is …

(a) 0 % 

(b) 4.4 % 

(c) 16 % 

(d) 8.4 %

Answer 1

(c) 16% 

Mg CO₃ → MgO + CO₂ ↑ 

Mg CO₃ : (1 × 24) + (1 × 12) + (3 × 16) = 84 g 

CO₂ : (1 × 12) + (2 × 16) 44g 

100% pure 84 g MgCO₃ on heating gives 44 g CO₂ 

Given that I g of MgCO₃ on heating gives 0.44 g CO₂ 

Therefore, 84 g MgCO sample on heating gives 36.96 g CO₂ = 100% 

Percentage of purity of the sample = 100% / 44gCO₂ × 36.96 g CO₂ = 84%

Percentage of impurity = 16%