Empirical formula of $\mathrm{C}_8\mathrm{H}_{18}$ is $\mathrm{C}_4\mathrm{H}_9$ . Empirical formula is the simplest ratio of elements present in the compound. In $\mathrm{C}_8\mathrm{H}_{18}$ the ratio of $\mathrm{C}$ and $\mathrm{H}$ is $8:18$ , To get simple ratio divide by 2 . $\therefore$ Empirical formula $=4:9$ .