Oxyacid of phosphorus that can reduce `AgNO_3` to silver is

Options

A. `H_3PO_4`
B. `H_4P_2O_7`
C. `H_3PO_3`
D. `HPO_3`

Answer

To determine which oxyacid of phosphorus can reduce AgNO3 to silver, we need to analyze the structures of the given oxyacids and identify the presence of a P-H bond, as this bond is associated with strong reducing properties.

### Step-by-Step Solution:

1. **Identify the Oxyacids of Phosphorus**: The oxyacids of phosphorus we need to consider are:
- H3PO4 (Phosphoric acid)
- H4P2O7 (Pyrophosphoric acid)
- H3PO3 (Phosphorous acid)
- HPO3 (Metaphosphoric acid)

2. **Analyze H3PO4**:
- Structure: P(=O)(OH)3
- There is no P-H bond present in H3PO4.
- Conclusion: H3PO4 cannot act as a reducing agent.

**Hint**: Check for the presence of P-H bonds in the structure.

3. **Analyze H4P2O7**:
- Structure: This is a dimer of H3PO4 and does not contain P-H bonds.
- Conclusion: H4P2O7 cannot act as a reducing agent.

**Hint**: Consider the structural composition of the acid and look for P-H bonds.

4. **Analyze H3PO3**:
- Structure: P(=O)(OH)2(H)
- Here, we can see that there is a P-H bond present.
- Conclusion: H3PO3 can act as a strong reducing agent due to the presence of the P-H bond.

**Hint**: Identify the functional groups and bonds that contribute to reducing properties.

5. **Analyze HPO3**:
- Structure: P(=O)(OH)(=O)
- There is no P-H bond present in HPO3.
- Conclusion: HPO3 cannot act as a reducing agent.

**Hint**: Look for the specific bonds that indicate reducing capabilities.

6. **Final Conclusion**:
- The oxyacid of phosphorus that can reduce AgNO3 to silver is H3PO3 (Phosphorous acid).

### Answer:
The correct option is **H3PO3**.