WAEC Chemistry 2010 Paper

Concept:A Group III element has a valency of 3, while oxygen has a valency of 2.The formula of the oxide is obtained by crossing over these valencies.Explanation:Valency of the metal $X = 3$.Valency of oxygen $O = 2$.Write the symbols side by side: $X$ and $O$.Cross-multiply the valencies: the number of $X$ atoms becomes 2, and the number of $O$ atoms becomes 3.Therefore, the chemical formula of the oxide is $\mathrm{X}_2\mathrm{O}_3$.This matches the pattern seen in oxides such as $\mathrm{Al}_2\mathrm{O}_3$ and $\mathrm{Fe}_2\mathrm{O}_3$.Answer:C. $\mathrm{X}_2\mathrm{O}_3$