BECE Science 2012 Paper

Concept:The formula of an ionic compound is found by balancing the positive and negative charges of its ions.Explanation:The valency of aluminium is $3$.The valency of oxygen is $2$.In $\mathrm{Al}_x\mathrm{O}_y$, the total positive charge is $+3x$.The total negative charge is $-2y$.For a neutral compound, $3x = 2y$.The smallest whole-number solution is $x = 2$ and $y = 3$.So the formula is $\mathrm{Al}_2\mathrm{O}_3$.Answer:The values of $x$ and $y$ are $2$ and $3$ respectively.Correct option: D.