WAEC Chemistry 2018 Paper

Concept:The molecular formula is a whole-number multiple of the empirical formula.Explanation:The empirical formula is given as $\mathrm{CH}_2$.Let the molecular formula be $(\mathrm{CH}_2)_n$, where $n$ is the number of empirical units in one molecule.The molar mass of the empirical unit is $12 + 2(1) = 14\ \text{gmol}^{-1}$.Therefore, the molar mass of the molecular formula is $14n\ \text{gmol}^{-1}$.Since the given molar mass is $42\ \text{gmol}^{-1}$, we write $14n = 42$.Solving for $n$ gives $n = 3$.Hence, the molecular formula is $(\mathrm{CH}_2)_3 = \mathrm{C}_3\mathrm{H}_6$.Answer:$\mathrm{C}_3\mathrm{H}_6$, which is option C.