Weight percentage of hydrogen present in the lightest hydrocarbon is

• The lightest hydrocarbon is known as methane having a chemical formula ${\mathrm{CH}}_4$.
• Methane gas contains carbon and hydrogen in it.
• The molar mass of C =12 and H = 1.

So, the molar mass of ${\mathrm{CH}}_4$ is $=12+4\left(1\right)=16$

The ratio of carbon: hydrogen is $12/4=3:1$

weight $\%$ of hydrogen = molar mass of hydrogen/molar mass of methane $\times 100$

wt. $\%=\frac{4}{16}\times 100=25\%$

So, the weight percentage of hydrogen is $25\%$ and carbon is $75\%$.