Excess of concentrated sodium hydroxide can separate mixture of

Options

A. `Al^(3+)` and `Cr^(3+)`
B. `Cr^(3+)` and `Fe^(3+)`
C. `Al^(3+)` and `Zn^(3+)`
D. `Zn^(2+)` and `Pb^(2+)`

Explanation

To solve the question regarding which pairs can be separated by excess concentrated sodium hydroxide, we need to analyze the behavior of different metal ions in the presence of sodium hydroxide.

### Step-by-Step Solution:

1. **Identify the Metal Ions**: The metal ions mentioned in the question are Aluminium (Al³⁺), Chromium (Cr³⁺), Iron (Fe³⁺), Zinc (Zn²⁺), and Lead (Pb²⁺).

2. **Understand the Reaction with Sodium Hydroxide**:
- When metal ions react with sodium hydroxide (NaOH), they can either form soluble complexes or precipitates.
- For example, Aluminium (Al³⁺) and Chromium (Cr³⁺) can form soluble complexes with excess NaOH, while Iron (Fe³⁺) typically forms a precipitate.

3. **Reactions of Each Ion**:
- **Chromium (Cr³⁺)**:
- With NaOH: Cr³⁺ + 3OH⁻ → Cr(OH)₃ (precipitate)
- With excess NaOH: Cr(OH)₃ + OH⁻ → [Cr(OH)₄]⁻ (soluble complex)
- **Iron (Fe³⁺)**:
- With NaOH: Fe³⁺ + 3OH⁻ → Fe(OH)₃ (precipitate)
- Does not form a soluble complex with excess NaOH.
- **Aluminium (Al³⁺)**:
- With NaOH: Al³⁺ + 3OH⁻ → Al(OH)₃ (precipitate)
- With excess NaOH: Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (soluble complex)
- **Zinc (Zn²⁺)**:
- With NaOH: Zn²⁺ + 2OH⁻ → Zn(OH)₂ (precipitate)
- With excess NaOH: Zn(OH)₂ + 2OH⁻ → [Zn(OH)₄]²⁻ (soluble complex)
- **Lead (Pb²⁺)**:
- With NaOH: Pb²⁺ + 2OH⁻ → Pb(OH)₂ (precipitate)
- Does not form a soluble complex with excess NaOH.

4. **Analysis of Pairs**:
- **Al³⁺ and Cr³⁺**: Both form soluble complexes with excess NaOH.
- **Cr³⁺ and Fe³⁺**: Cr³⁺ forms a soluble complex, while Fe³⁺ forms a precipitate. These can be separated.
- **Al³⁺ and Zn²⁺**: Both form soluble complexes.
- **Zn²⁺ and Pb²⁺**: Both form precipitates and do not form soluble complexes.

5. **Conclusion**: The pair that can be separated by excess concentrated sodium hydroxide is **Chromium (Cr³⁺) and Iron (Fe³⁺)**, as Cr³⁺ forms a soluble complex while Fe³⁺ forms a precipitate.

### Final Answer:
Excess of concentrated sodium hydroxide can separate the mixture of **Chromium (Cr³⁺) and Iron (Fe³⁺)**.