When `NH_(4)Cl` is not used together with `NH_(4)OH` in group-III reagent which of the following cation will not be precipitated?

Options

A. `Fe^(2+)`
B. `Cr^(3+)`
C. `Zn^(2+)`
D. `NH_(3)` soln.

Explanation

To solve the question of which cation will not be precipitated when `NH4Cl` is not used together with `NH4OH` in group-III reagent, we need to analyze the behavior of each cation in the presence of ammonium hydroxide (NH4OH).

### Step-by-Step Solution:

1. **Identify the Cations**: The cations given are:
- Fe²⁺ (Iron II)
- Cr³⁺ (Chromium III)
- Zn²⁺ (Zinc II)
- NH₃ (Ammonia solution)

2. **Understand the Role of Ammonium Hydroxide**: Ammonium hydroxide (NH4OH) acts as a reagent that can precipitate certain metal hydroxides when mixed with solutions containing metal cations.

3. **Reactions with Ammonium Hydroxide**:
- **Fe²⁺ with NH4OH**:
\[ \text{Fe}^{2+} + 2 \text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_2 \downarrow (\text{precipitate}) \]
Iron II hydroxide (Fe(OH)₂) precipitates out of solution.

- **Cr³⁺ with NH4OH**:
\[ \text{Cr}^{3+} + 3 \text{NH}_4\text{OH} \rightarrow \text{Cr(OH)}_3 \downarrow (\text{precipitate}) \]
Chromium III hydroxide (Cr(OH)₃) also precipitates.

- **Zn²⁺ with NH4OH**:
\[ \text{Zn}^{2+} + 2 \text{NH}_4\text{OH} \rightarrow \text{Zn(OH)}_2 \downarrow (\text{precipitate}) \]
Zinc hydroxide (Zn(OH)₂) precipitates as well.

4. **Ammonia Solution (NH₃)**:
- Ammonia solution (NH₃ in water) is essentially ammonium hydroxide (NH4OH). Therefore, when NH₃ is present, it does not precipitate because it is in equilibrium with NH4OH, which is soluble.

5. **Conclusion**: Among the cations listed, the only one that does not precipitate when NH4OH is present is ammonia solution (NH₃). Therefore, the answer to the question is:
- **Ammonia solution (NH₃)** will not be precipitated.

### Final Answer:
The cation that will not be precipitated is **Ammonia solution (NH₃)**.

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