Air contains 21% of oxygen by weight. What weight of air is required to burn 50 grams of coal which contains only 50% combustible material ?

Weight of Coal = 50 gms

percentage of Combustible material in coal = 50%

Weight of combustible material of coal =$\frac{50}{100}\times 50=25\mathrm{gms}$

$$\begin{gathered} \mathrm{C} + {\mathrm{O}}_2\to {\mathrm{CO}}_2 \\ 12\mathrm{g} 32\mathrm{g} \\ 25\mathrm{g} \mathrm{X} \mathrm{g} \end{gathered}$$

Weight of oxygen in air = $\left(\frac{25\times 32}{12}\right)\mathrm{g}$

weight % of oxygen in air = 21%

weight of air required = $\left(\frac{25\times 32}{12}\times \frac{100}{21}\right)\mathrm{g}$
 weight of air required = 317g