JAMB Chemistry 2014 Paper

Concept:Qualitative analysis: NaOH produces a white precipitate; if it is insoluble in excess NaOH, the cation is not amphoteric.Explanation:A few drops of NaOH gives a white precipitate for $\mathrm{Zn}^{2+}$, $\mathrm{Pb}^{2+}$, $\mathrm{Ca}^{2+}$, and $\mathrm{Al}^{3+}$. However, $\mathrm{Zn}^{2+}$, $\mathrm{Pb}^{2+}$, and $\mathrm{Al}^{3+}$ form amphoteric hydroxides that dissolve in excess NaOH. $\mathrm{Ca}^{2+}$ forms $\mathrm{Ca(OH)_2}$, a white precipitate that is insoluble in excess NaOH. Therefore, the cation is calcium, $\mathrm{Ca}^{2+}$.Answer:C. $\mathrm{Ca}^{2+}$