Atoms and Molecules — Question 2

Given,

$\underset{\text{ 3 g}}{\text{C}} + \underset{\text{ 8 g}}{\text{O}_2} \longrightarrow \underset{\text{11 g}}{\text{CO}_2}$

When 3.0 g of carbon is burnt in 8.00 g of oxygen, 11.00 g of carbon dioxide is produced.

Now,

$\underset{\text{ 3 g}}{\text{C}} + \underset{\text{ 50 g}}{\text{O}_2} \longrightarrow \underset{\text{?}}{\text{CO}_2}$

Carbon dioxide contains carbon and oxygen in a fixed ratio by mass, which is 3:8. Hence, 3 g of carbon will react with 50 g of oxygen but only 8 g of it will be used in producing 11 g of CO₂ and 42 g [i.e., 50 g - 8 g] of oxygen will be left unused.

The above answer is governed by the law of constant proportions.