120g 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

Number of moles of C = Number of moles of CO₂ = $\frac{330}{44}$ moles

Number of moles of H = 2 * no. of moles of H₂O = $\left(\frac{270}{18}*2\right)$ moles

 Mass of C = $\frac{330}{44}*12$ gm = 90 gm

 Mass of H = $\frac{270}{18}*2*1gm=30gm$

$\begin{array}{l}\mathrm{\%}ofC=\frac{90}{120}*100\mathrm{\%}=75\mathrm{\%}\\ \mathrm{\%}ofH=\frac{30}{120}*100\mathrm{\%}=25\mathrm{\%}\end{array}$