WAEC Chemistry 2012 Paper

Concept:Oxidation number is the charge an atom would have if electrons in each bond are assigned to the more electronegative atom. For a complex ion, the sum of the oxidation numbers of all atoms equals the overall charge on the ion.Explanation:Let the oxidation number of iron be $x$.The cyanide ion, $\mathrm{CN}^-$, always has an oxidation number of $-1$.In $[\mathrm{Fe}(\mathrm{CN})_6]^{3-}$, there are six cyanide ligands.So the total contribution from all cyanide ions is $6 \times (-1) = -6$.The overall charge on the complex ion is $-3$.Hence, the equation is: $x + (-6) = -3$.Simplify: $x - 6 = -3$.Add $6$ to both sides: $x = -3 + 6$.Therefore, $x = +3$.This means the oxidation number of $\mathrm{Fe}$ is $+3$.Answer:The oxidation number of Fe in $[\mathrm{Fe}(\mathrm{CN})_6]^{3-}$ is $+3$, i.e. option A.