Cations are classified into varius group on the basis of their behaviour against some reagents .The group reagent used for the classifaction of most common cation are `HCI,H_(2)S,NH_(4)OH,(NH_(4))_(2)CO_(3)`. Classification is based on whether a cation reacts with these reagents by the formation of precipitates or not .
An aqucous solution which is sightly acids contains cattions `Fe^(2+),Zn^(2+) and Cu^(2+)`. THe rengent added in excess to this solution would identify the separate `Fe^(2+)` ion in one step is

Options

A. `2M HCI`
B. `6M NH_(3)`
C. `6M NaOH`
D. `H_(2)S gas `

Explanation

To solve the question regarding the identification of the separate Fe²⁺ ion in a slightly acidic solution containing cations Fe²⁺, Zn²⁺, and Cu²⁺, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Cations Present**:
The solution contains three cations: Fe²⁺, Zn²⁺, and Cu²⁺. We need to identify the Fe²⁺ ion specifically.

2. **Choosing the Right Reagent**:
We need a reagent that can selectively react with Fe²⁺ and differentiate it from Zn²⁺ and Cu²⁺.

3. **Using Ammonium Hydroxide (NH₄OH)**:
When ammonium hydroxide is added to the solution, it can precipitate certain cations. However, in the case of Fe²⁺, it will not precipitate immediately in acidic conditions.

4. **Adding Ammonium Hydroxide in Excess**:
If we add an excess of 6 M NH₄OH to the solution, it will convert Fe²⁺ to Fe(OH)₂, which is a green precipitate. However, to specifically identify Fe²⁺, we need to oxidize it to Fe³⁺.

5. **Oxidation of Fe²⁺ to Fe³⁺**:
To achieve this, we can add a strong oxidizing agent like H₂O₂ or KMnO₄. However, in this case, we will use NH₄OH in excess, which will lead to the formation of Fe(OH)₃ (reddish-brown precipitate) when Fe²⁺ is oxidized to Fe³⁺.

6. **Formation of Precipitate**:
The reddish-brown precipitate of Fe(OH)₃ confirms the presence of Fe³⁺, indicating that Fe²⁺ was present in the solution.

7. **Final Confirmation**:
The other cations, Zn²⁺ and Cu²⁺, will form soluble complexes with NH₄OH (Zn²⁺ forms a colorless complex and Cu²⁺ forms a blue complex), thus confirming that the precipitate formed is indeed from Fe²⁺.

### Conclusion:
The reagent that should be added in excess to identify the Fe²⁺ ion in the solution is **6 M NH₄OH**.

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