This question tests your understanding of qualitative analysis of cations using group reagents. Ammoniacal H₂S is a specific reagent used to precipitate certain metal sulfides.

Key Concept: In qualitative analysis, cations are separated into groups based on their precipitation behavior. Ammoniacal H₂S is the group reagent for Group IV cations. This group includes:

• Zinc (Zn²⁺)
• Manganese (Mn²⁺)
• Nickel (Ni²⁺)
• Cobalt (Co²⁺)

These ions form sulfide precipitates in basic medium (provided by NH₄OH). The reaction for Zinc is:

${\mathrm{Zn}}^{\left(\mathrm{II}\right)}+H_{2}S\to \mathrm{ZnS}▒+2H^{+}$

Now, let's analyze the given options:

• Al(III): Precipitates as Al(OH)₃ with NH₄OH (Group III). It does not precipitate with H₂S.
• Mg(II): Does not precipitate with any group reagent. It remains in solution (Group VI).
• Zn(II): Precipitates as white ZnS with ammoniacal H₂S (Group IV).
• Fe(III): Precipitates as Fe(OH)₃ with NH₄OH (Group III). It does not precipitate with H₂S.

Final Answer: Zn(II) is the metal ion that precipitates as a sulfide with ammoniacal H₂S.

Related Topics & Formulae

Group IV Precipitation: Cations of Group IV are precipitated as their sulfides in ammoniacal solution. The general reaction is:

$M^{2+}+H_{2}S\to \mathrm{MS}▒+2H^{+}$

(where M²⁺ is a Group IV cation like Zn²⁺, Ni²⁺, etc.)

Why ammoniacal medium? H₂S is a weak acid. In acidic medium, the concentration of S^2- ions is very low, and only the most insoluble sulfides (like Group II: HgS, CuS, etc.) precipitate. In basic medium (like NH₄OH), the concentration of S^2- ions increases, allowing less insoluble sulfides (Group IV) to precipitate.