Question

The reagent \(NH_4Cl\) and aqueous \(NH_3\) will precipitate:

• A. Ca\(^{2+}\)
• B. Al\(^{3+}\)
• C. Mg\(^{2+}\)
• D. Zn\(^{2+}\)

Hint:

\(NH_4Cl + NH_3\) buffer → selectively precipitates Group III cations (Al\(^{3+}\), Fe\(^{3+}\), Cr\(^{3+}\)).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In qualitative inorganic analysis, cations are divided into groups based on the reagents that cause them to precipitate as specific salts. The mixture of \( \text{NH}_{4}\text{Cl} \) and \( \text{NH}_{4}\text{OH} \) (aqueous \( \text{NH}_{3} \)) is the Group III reagent.
Step 2: Detailed Explanation:
1. Group III Cations: This group includes \( \text{Fe}^{3+}, \text{Al}^{3+}, \) and \( \text{Cr}^{3+} \). They are precipitated as their hydroxides.
2. Role of Reagents: \( \text{NH}_{4}\text{OH} \) provides hydroxide ions (\( \text{OH}^{-} \)). \( \text{NH}_{4}\text{Cl} \) is added to suppress the ionization of \( \text{NH}_{4}\text{OH} \) (common ion effect).
3. Solubility Product: This ensures that the concentration of \( \text{OH}^{-} \) is only high enough to exceed the \( \text{K}_{\text{sp}} \) of Group III hydroxides (which are very low) but not Group IV or V hydroxides.
4. Observations:
- \( \text{Al}^{3+} \) precipitates as a gelatinous white precipitate of \( \text{Al(OH)}_{3} \).
- \( \text{Ca}^{2+} \) (Group V) and \( \text{Mg}^{2+} \) (Group VI) do not precipitate with this reagent.
- \( \text{Zn}^{2+} \) (Group IV) precipitates as a sulfide (\( \text{ZnS} \)) using \( \text{H}_{2}\text{S} \) in basic medium.
Step 3: Final Answer:
The reagent will precipitate \( \text{Al}^{3+} \).