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

${\text{C}}_{\text{x}}{\text{H}}_{\text{y}}+\left[\text{x}+\frac{\text{y}}{4}\right]{\text{O}}_2⟶\text{xC}{\text{O}}_2+\frac{\text{y}}{2}{\text{H}}_2\text{O}$

Moles of ${\mathrm{CO}}_2$ $=\frac{3\text{.}08}{44}=0\text{.}07\text{ moles}=\text{x}$

Moles of ${\mathrm{H}}_2\mathrm{O}$ $=\frac{0\text{.}72}{18}=0\text{.}04\text{ moles}=\frac{\text{y}}{2}$

$\mathrm{x} = 0.07 \mathrm{g}$ atoms of carbon

$\mathrm{y} = 0.08 \mathrm{g}$ atoms of hydrogen

$$\begin{gathered} \mathrm{x} : \mathrm{y} \\ 0.07 0.08 \\ 7 8 \end{gathered}$$

Empirical formula becomes ${\mathrm{C}}_7{\mathrm{H}}_8$.