Which of the following pairs of cations cannot be separated by using an `NH_(3)` solution?

Options

A. `Pb^(2+),Zn^(2+)`
B. `Pb^(2+),Cu^(2+)`
C. `Zn^(2+),Cu^(2+)`
D. `Al^(3+),Ag^(+)`

Answer

To determine which pair of cations cannot be separated using an ammonia solution, we will analyze the interactions of each pair with ammonia (NH₃) and identify whether they form soluble or insoluble complexes.

### Step-by-Step Solution:

1. **Identify the Cations**: The pairs of cations given are:
- (i) Pb²⁺ and Zn²⁺
- (ii) Pb²⁺ and Cu²⁺
- (iii) Zn²⁺ and Cu²⁺
- (iv) Al³⁺ and Ag⁺

2. **Reactions with Ammonia**:
- **Pb²⁺ (Lead II) with NH₃**:
- Pb²⁺ + NH₄OH → Pb(OH)₂ (White precipitate)
- Lead hydroxide is insoluble in water.
- **Zn²⁺ (Zinc II) with NH₃**:
- Zn²⁺ + NH₃ → [Zn(NH₃)₄]²⁺ (Colorless soluble complex)
- Zinc forms a soluble complex with ammonia.
- **Cu²⁺ (Copper II) with NH₃**:
- Cu²⁺ + NH₃ → [Cu(NH₃)₄]²⁺ (Blue soluble complex)
- Copper also forms a soluble complex with ammonia.
- **Al³⁺ (Aluminum III) with NH₃**:
- Al³⁺ + NH₄OH → Al(OH)₃ (White precipitate)
- Aluminum hydroxide is insoluble in water.
- **Ag⁺ (Silver I) with NH₃**:
- Ag⁺ + NH₃ → [Ag(NH₃)₂]⁺ (Soluble complex)
- Silver forms a soluble complex with ammonia.

3. **Analyze Each Pair**:
- **Pair (i) Pb²⁺ and Zn²⁺**:
- Pb²⁺ is insoluble (precipitate), Zn²⁺ is soluble (complex). They can be separated.
- **Pair (ii) Pb²⁺ and Cu²⁺**:
- Pb²⁺ is insoluble (precipitate), Cu²⁺ is soluble (complex). They can be separated.
- **Pair (iii) Zn²⁺ and Cu²⁺**:
- Both Zn²⁺ and Cu²⁺ form soluble complexes with NH₃. They cannot be separated.
- **Pair (iv) Al³⁺ and Ag⁺**:
- Al³⁺ is insoluble (precipitate), Ag⁺ is soluble (complex). They can be separated.

4. **Conclusion**: The pair of cations that cannot be separated by using an ammonia solution is **Zn²⁺ and Cu²⁺** because both form soluble complexes.

### Final Answer:
The pair of cations that cannot be separated by using an NH₃ solution is **Zn²⁺ and Cu²⁺**.