Which of the following is presipitated when an arsenate reacts withh a magnesium mixture?

Options

A. `MgHAsO_(3)`
B. `Mg_(2)AsO_(3)`
C. `MgNH_(4)AsO_(4)*6H_(2)O`
D. `Mg_(2)NH_(4)AsO_(4)*6H_(2)O`

Answer

To solve the question of which compound is precipitated when an arsenate reacts with a magnesium mixture, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components**:
- The arsenate ion is represented as \( \text{AsO}_4^{3-} \).
- The magnesium mixture typically consists of magnesium chloride (\( \text{MgCl}_2 \)), ammonium chloride (\( \text{NH}_4\text{Cl} \)), and ammonia solution (\( \text{NH}_3 \) or \( \text{NH}_4\text{OH} \)).

2. **Write the Reaction**:
- When the magnesium mixture reacts with the arsenate, we can represent the reaction as:
\[ \text{MgCl}_2 + \text{NH}_4\text{Cl} + \text{NH}_3 + \text{AsO}_4^{3-} \rightarrow \text{MgNH}_4\text{AsO}_4 \]

3. **Formation of the Precipitate**:
- The product formed from this reaction is magnesium ammonium arsenate, which can be hydrated. The hydrated form is:
\[ \text{MgNH}_4\text{AsO}_4 \cdot 6\text{H}_2\text{O} \]
- This indicates that the precipitate formed is magnesium ammonium arsenate hexahydrate.

4. **Identify the Correct Option**:
- Among the given options, the correct one that corresponds to the precipitate formed is:
\[ \text{MgNH}_4\text{AsO}_4 \cdot 6\text{H}_2\text{O} \]
- This matches with option three.

5. **Conclusion**:
- Therefore, the final answer is that the precipitate formed when an arsenate reacts with a magnesium mixture is:
\[ \text{MgNH}_4\text{AsO}_4 \cdot 6\text{H}_2\text{O} \]
- Hence, the correct option is option three.