JAMB Chemistry 2024 Paper

Concept:Vapour density is half of the molecular mass of a compound.$Molecular\ mass = 2 \times Vapour\ density$Explanation:The empirical formula of the hydrocarbon is $XY$.Relative molar mass of $X$ is $72$, and that of $Y$ is $6$.Therefore, empirical formula mass $= 72 + 6 = 78$.For an organic compound, molecular mass is a whole-number multiple of the empirical formula mass.Taking the simplest case, molecular mass $= 78$.Using $Molecular\ mass = 2 \times Vapour\ density$:$78 = 2 \times Vapour\ density$$Vapour\ density = \frac{78}{2} = 39$Answer:$39$, which is option C.