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 $=4:9$ .