The addition of ammonium chloride to a solution containing ferric and magnesium ions is essential for selective precipitation of ferric hydroxide by aqueous ammonia.

Explanation

To understand the selective precipitation of ferric hydroxide in the presence of ammonium chloride when ferric and magnesium ions are both present, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding the Components**:
- We have a solution containing ferric ions (Fe³⁺) and magnesium ions (Mg²⁺).
- We will be adding ammonium chloride (NH₄Cl) and aqueous ammonia (NH₄OH) to this solution.

2. **Role of Ammonium Chloride**:
- The addition of ammonium chloride introduces NH₄⁺ ions into the solution.
- NH₄⁺ ions act as a common ion with respect to the ammonium hydroxide (NH₄OH) that will form when ammonia is added.

3. **Common Ion Effect**:
- The presence of the common ion (NH₄⁺) suppresses the ionization of ammonium hydroxide.
- This suppression reduces the concentration of OH⁻ ions in the solution, which is crucial for the precipitation of hydroxides.

4. **Selective Precipitation of Ferric Hydroxide**:
- Ferric hydroxide (Fe(OH)₃) can precipitate out of the solution when the concentration of OH⁻ ions reaches a certain level.
- The solubility product (Ksp) of ferric hydroxide is lower than that of magnesium hydroxide (Mg(OH)₂), meaning that ferric hydroxide precipitates at a lower concentration of OH⁻ compared to magnesium hydroxide.

5. **Preventing Magnesium Hydroxide Precipitation**:
- Since magnesium hydroxide has a higher Ksp, the reduced concentration of OH⁻ due to the common ion effect prevents the precipitation of magnesium hydroxide.
- Thus, only ferric hydroxide precipitates out of the solution when aqueous ammonia is added.

6. **Conclusion**:
- Therefore, the addition of ammonium chloride is essential for selectively precipitating ferric hydroxide while preventing the precipitation of magnesium hydroxide in a solution containing both ions.