Number of hydrogen atoms per molecules of a hydrocarbon A having 85.8% carbon is _________
(Given : molar mass of A = 84 g mol^-1)

$\begin{array}{r}\mathrm{\%}\text{ of }\mathrm{C}\frac{85.8}{12}=\frac{7.15}{7.15}=1\\ \mathrm{\%}\text{ of }\mathrm{H}\frac{14.2}{1}=\frac{14.2}{7.15}=2\end{array}$
Empirical formula $={\mathrm{CH}}_2$
Molecular formula $=(\mathrm{EF}{)}_{\mathrm{n}}\Rightarrow \mathrm{n}=\frac{\mathrm{MW}}{\mathrm{EFW}}=\frac{84}{14}=6$
$\mathrm{MF}={\left({\mathrm{CH}}_2\right)}_6={\mathrm{C}}_6{\mathrm{H}}_{12}$
Number of hydrogen atoms = 12