WAEC Chemistry 2017 Paper

Concept:The oxidation number of an atom in a polyatomic ion is calculated by making the sum of all oxidation numbers equal to the ion’s overall charge.Explanation:Let the oxidation number of $\mathrm{X}$ be $x$.Oxygen is usually assigned an oxidation number of $-2$.In $\mathrm{XO}_{4}^{3-}$, there are four oxygen atoms, so the total contribution from oxygen is $4 \times (-2) = -8$.Set up the equation: $x + (-8) = -3$.Solve for $x$: $x = -3 + 8 = +5$.Answer:The oxidation number of $\mathrm{X}$ is $+5$, which is option D.