Heating an ammoniacal solution of `MgSO_(4)` in the presence of `NH_(4)Cl` and `Na_(2)HPO_(4)` causes the precipitation of:

Options

A. `Mg (HPO_(4))`
B. `Mg (NH_(4))PO_(4)`
C. `MgCl_(2)`
D. `Mg (NH_(4))_(2) (PO_(4))_(2)`

Explanation

To solve the question regarding the precipitation that occurs when heating an ammoniacal solution of MgSO₄ in the presence of NH₄Cl and Na₂HPO₄, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**:
We have three main reactants in this scenario:
- Magnesium sulfate (MgSO₄)
- Ammonium chloride (NH₄Cl)
- Sodium hydrogen phosphate (Na₂HPO₄)

2. **Understand the Reaction Conditions**:
The solution is ammoniacal, meaning it contains ammonia (NH₃) which can influence the solubility of certain compounds. Additionally, heating the solution can promote reactions that lead to precipitation.

3. **Write the Possible Reaction**:
When these reactants are mixed, we can expect a reaction between magnesium ions (Mg²⁺) from MgSO₄ and phosphate ions (PO₄³⁻) from Na₂HPO₄. The ammonium ions (NH₄⁺) from NH₄Cl will also play a role in the formation of a complex.

The key reaction can be represented as:
\[ \text{Mg}^{2+} + \text{NH}_4^+ + \text{PO}_4^{3-} \rightarrow \text{MgNH}_4\text{PO}_4 \text{ (s)} \]

4. **Identify the Precipitate**:
The compound formed in the reaction is magnesium ammonium phosphate, which has the formula MgNH₄PO₄. This compound is known to precipitate out of solution as a white solid.

5. **Conclusion**:
Therefore, upon heating the ammoniacal solution of MgSO₄ in the presence of NH₄Cl and Na₂HPO₄, the precipitate formed is magnesium ammonium phosphate (MgNH₄PO₄).

### Final Answer:
The precipitation of magnesium ammonium phosphate (MgNH₄PO₄) occurs.