JAMB Chemistry 2010 Paper

Concept:The brown ring test is a confirmatory test for nitrate ions, $\mathrm{NO}_3^-$.Explanation:Freshly prepared iron(II) tetraoxosulphate(VI), $\mathrm{FeSO}_4$, is added to the unknown solution.A little dilute tetraoxosulphate(VI) acid, $\mathrm{H}_2\mathrm{SO}_4$, is also added.Concentrated $\mathrm{H}_2\mathrm{SO}_4$ is then carefully poured down the side of the test tube.This strong acid reduces nitrate ions to nitrogen monoxide, $\mathrm{NO}$.The nitrogen monoxide reacts with the iron(II) complex to form a brown nitrosoferrous compound.A brown ring appears at the junction between the two liquid layers.Only nitrate ions produce this characteristic brown ring under these conditions.Answer:The presence of the nitrate ion, $\mathrm{NO}_3^-$, is confirmed. Therefore, the correct option is D.