120g of an organic compound that contains only carbon and hydrogen gives 330g of $C{O}_2$ and 270 g of water on complete combustion. The percentage of carbon and hydrogen respectively are

$CxHy+\left(x+\frac{y}{4}\right)O_2\to x{CO}_2+\frac{y}{2}{H}_{2}O$

$\text{ Mole of }C{O}_2=\frac{330}{44}=7.5$

$\text{ Mole of }{\mathrm{H}}_2\mathrm{O}=\frac{270}{18}=15$

Mass of carbon = Mole of carbon $\times$ atomic-mass

= 7.5 X 12 = 90g

% of carbon =$\frac{wt of C{O}_2 X 12}{44}X\frac{100}{wt of o.comp}$

% of hydrogen = 100 – 75 = 25%