A gaseous hydrocarbon gives upon combustion 0.72 g of water and 3.08 g of ${\mathrm{CO}}_2$. The empirical formula of the hydrocarbon is

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

Moles of  ${\mathrm{H}}_2\mathrm{O}=\frac{0.72}{18}=0.04$

${\mathrm{CO}}_2=\frac{3.08}{44}=0.07$

$\frac{\mathrm{y}}{2}=0.04\times 100\mathrm{or}=8$

$\mathrm{x}=0.07\times 100=7$

Empirical formula is  ${\mathrm{C}}_7{\mathrm{H}}_8$