120 gm of an organic compound that contains only carbon and hydrogen gives 330g of CO₂ and 270g of water on complete combustion. The percentage of carbon and hydrogen, respectively are.

Given mass of organic compound = 120 gm
mass of CO₂(g) = 330 g

$$\begin{gathered} \text{ mass of }{\mathrm{H}}_2\mathrm{O}\left(\ell \right)=270\mathrm{g} \\ \text{ mass of carbon }={\mathrm{n}}_{{\mathrm{CO}}_2}\times 12 \end{gathered}$$

$=\frac{330}{44}\times 12=90\mathrm{g}$

$\mathrm{\%}\text{ of carbon }=\frac{90}{120}\times 100=75\mathrm{\%}$

$\text{ mass of hydrogen }={\mathrm{n}}_{{\mathrm{H}}_2\mathrm{O}}\times 2$

$=\frac{270}{18}\times 2=30\mathrm{g}$

$\mathrm{\%}\text{ of hydrogen }=\frac{30}{120}\times 100=25\mathrm{\%}$