WAEC Chemistry 2013 Paper

Concept:Use the criss-cross method to balance the valencies of the two ions and get the neutral formula.Explanation:M is a trivalent metal, so its valency is $M^{3+}$.Y is a divalent non-metal, so its valency is $Y^{2-}$.To make the compound neutral, the total positive charge must equal the total negative charge.The lowest common multiple of $3$ and $2$ is $6$.Hence, $2$ atoms of $M$ give $2 \times (+3) = +6$.And $3$ atoms of $Y$ give $3 \times (-2) = -6$.Therefore, the correct formula is $\mathrm{M_2Y_3}$.Answer:$\mathrm{M_2Y_3}$ (Option A).