WAEC Chemistry 2010 Paper

Concept:Oxidation number is assigned using known charges, and the change reflects the electron gain by manganese.Explanation:In $\mathrm{MnO}_4^-$, let the oxidation number of Mn be $x$.Oxygen has an oxidation number of $-2$, and the total charge on the ion is $-1$.Therefore, $x + 4(-2) = -1$.This gives $x - 8 = -1$.So, $x = +7$.Thus, Mn is in the $+7$ oxidation state in $\mathrm{MnO}_4^-$.On the product side, the species is $\mathrm{Mn}^{2+}$, so the oxidation number of Mn is $+2$.Hence, the oxidation number of manganese changes from $+7$ to $+2$.Answer:The oxidation number changes from $+7$ to $+2$.