WAEC Chemistry 2017 Paper

Concept:The empirical formula is obtained by converting the percentage composition of each element into moles and then simplifying the mole ratio.Explanation:The hydrocarbon contains only carbon and hydrogen.Since hydrogen is $14.3\%$, the percentage of carbon is $100\% - 14.3\% = 85.7\%$.In a $100\,g$ sample, the masses are $85.7\,g$ of carbon and $14.3\,g$ of hydrogen.Moles of carbon are $\frac{85.7}{12.0} = 7.14$ mol.Moles of hydrogen are $\frac{14.3}{1.0} = 14.3$ mol.Divide both mole values by the smallest value, $7.14$.The mole ratio becomes $C : H = \frac{7.14}{7.14} : \frac{14.3}{7.14} = 1 : 2$.Thus, the simplest whole-number ratio of carbon to hydrogen is $1 : 2$.Answer:The empirical formula of the hydrocarbon is $\mathrm{CH}_2$, which is option B.