JAMB Chemistry 2011 Paper

Concept:Concentrated $\mathrm{HNO}_3$ is a strong oxidizing agent that can oxidize $\mathrm{Fe}^{2+}$ ions to $\mathrm{Fe}^{3+}$ ions, which produce a brown solution.Explanation:$\mathrm{Fe}^{2+}$ solutions are usually pale green.When concentrated $\mathrm{HNO}_3$ is added and the mixture is boiled, $\mathrm{Fe}^{2+}$ is oxidized to $\mathrm{Fe}^{3+}$.The $\mathrm{Fe}^{3+}$ ions formed give a characteristic brown solution.$\mathrm{Pb}^{2+}$ and $\mathrm{Cu}^{2+}$ do not give this brown colour with concentrated $\mathrm{HNO}_3$ under these conditions.If the original cation were $\mathrm{Fe}^{3+}$, the solution would already appear brown without needing to add $\mathrm{HNO}_3$.Therefore, the cation initially present in the unknown solution is $\mathrm{Fe}^{2+}$.Answer:D. $\mathrm{Fe}^{2+}$