An aqueous solution of a metal salt, M. gives a white precipitate with NaOH which dissolves in excess NaOH…

Analysis of the Question

This question tests your knowledge of qualitative analysis, specifically the reactions of metal cations with sodium hydroxide (NaOH) and aqueous ammonia ($NH_3$). We need to identify a cation that forms a white precipitate with both reagents and, crucially, is soluble in the excess of both reagents.

Concept Explanation

In qualitative analysis, we use reagents like NaOH and $NH_3$ to identify metal ions based on the solubility of their hydroxides.

1. Reaction with NaOH: Many metal ions form white precipitates (hydroxides) with NaOH. However, only a few are amphoteric (meaning they dissolve in excess NaOH to form complex ions). Common examples include $Zn^{2+}$, $Al^{3+}$, and $Pb^{2+}$.
2. Reaction with Aqueous Ammonia: When $NH_3$ is added, it acts as a weak base. Most metal hydroxides precipitate. The key differentiator here is the ability to form a soluble ammine complex. • $Al^{3+}$: Forms a white precipitate that is insoluble in excess ammonia. • $Pb^{2+}$: Forms a white precipitate that is insoluble in excess ammonia. • $Zn^{2+}$: Forms a white precipitate ($Zn(OH)_2$) that dissolves in excess ammonia to form a colorless complex ion, $[Zn(NH_3)_4]^{2+}$.

Step-by-Step Solution

• Step 1: Identify the cation that forms a white precipitate with NaOH. All the options ($Zn^{2+}$, $Ca^{2+}$, $Al^{3+}$, $Pb^{2+}$) form white precipitates with NaOH.
• Step 2: Check solubility in excess NaOH. $Zn^{2+}$, $Al^{3+}$, and $Pb^{2+}$ are soluble in excess NaOH. $Ca^{2+}$ is not.
• Step 3: Check solubility in excess aqueous ammonia. • $Al^{3+}$: Precipitate remains (insoluble). • $Pb^{2+}$: Precipitate remains (insoluble). • $Zn^{2+}$: The precipitate dissolves in excess ammonia due to the formation of the tetraamminezinc(II) ion: $Zn(OH)_{2(s)} + 4NH_{3(aq)} \rightarrow [Zn(NH_3)_4]^{2+}_{(aq)} + 2OH^-_{(aq)}$

Since only $Zn^{2+}$ satisfies both conditions (dissolving in excess NaOH and excess ammonia), it is the correct cation.

Conclusion

The cation that exhibits these specific amphoteric and complex-forming properties is Zinc ($Zn^{2+}$).

Exam Tip

Remember the "Zinc-Copper-Silver" rule for ammonia: These three metals form soluble complexes with excess ammonia. If you see a question about a precipitate dissolving in excess ammonia, always look for these transition metals!