WAEC Chemistry 2016 Paper

Concept:For a neutral compound, the sum of the oxidation numbers of all atoms is zero. Use this rule to find the oxidation number of sulphur.Explanation:Consider $\mathrm{H}_2\mathrm{SO}_3$.Let the oxidation number of sulphur be $x$.Hydrogen has an oxidation number of $+1$.Oxygen has an oxidation number of $-2$.Write the equation for the total charge:$$2(+1) + x + 3(-2) = 0$$Simplify the terms:$$2 + x - 6 = 0$$$$x - 4 = 0$$$$x = +4$$Therefore, sulphur has oxidation number $+4$ in $\mathrm{H}_2\mathrm{SO}_3$.Answer:Option B: $\mathrm{H}_2\mathrm{SO}_3$