JAMB Chemistry 2020 Paper

Concept:The percentage composition is used to find the simplest whole-number mole ratio of $M$ to $Cl$ in the chloride.Explanation:$M$ makes up $20.22\%$ of the chloride by mass.Therefore, the mass percentage of chlorine is:$100\% - 20.22\% = 79.78\%$.Using the relative atomic masses of $M = 27$ and $Cl = 35.5$, calculate the moles in 100 g of the compound:Moles of $M = \frac{20.22}{27} = 0.75$ mol.Moles of $Cl = \frac{79.78}{35.5} = 2.25$ mol.Divide both values by the smallest value, $0.75$:$M:Cl = \frac{0.75}{0.75} : \frac{2.25}{0.75} = 1:3$.Thus, the simplest formula is $MCl_3$.Since only the percentage composition is given, the formula obtained is the empirical formula.Answer:The formula of the chloride is $\mathrm{MCl}_3$ (option C).