The formula of chloride of a metal M is `MCl_(3)`, then the number of oxygen atoms in the formula of the phosphate of metal M will be `"______________"`.

Explanation

To determine the number of oxygen atoms in the formula of the phosphate of metal M, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Chloride Formula**: We are given that the formula of the chloride of metal M is \( MCl_3 \).

2. **Determine the Valency of Metal M**: In the formula \( MCl_3 \), chlorine (Cl) has a valency of 1. Since there are three chlorine atoms, the total positive charge from metal M must balance the total negative charge from the three chlorine atoms. Therefore, the valency of metal M must be 3. This means that \( M \) has a valency of +3.

3. **Identify the Phosphate Ion**: The formula for the phosphate ion is \( PO_4^{3-} \). The phosphate ion has a valency of -3.

4. **Combine Metal M with Phosphate Ion**: Since the valency of metal M is +3 and the valency of the phosphate ion is -3, they can combine in a 1:1 ratio to form the compound. Thus, the formula for the phosphate of metal M will be \( MPO_4 \).

5. **Count the Oxygen Atoms**: In the formula \( MPO_4 \), there are 4 oxygen atoms.

### Final Answer:
The number of oxygen atoms in the formula of the phosphate of metal M is **4**.

---