The maximum number of reducing hydrogens are contained in which of the following molecule/s ?

Options

A. `H_3PO_3`
B. `H_3PO_2`
C. `H_4P_2O_7`
D. `H_4PO_4`

Answer

To determine which of the given compounds contains the maximum number of reducing hydrogens, we need to analyze the structures of the compounds and count the number of P-H bonds present in each. Reducing hydrogens are typically associated with P-H bonds in phosphorus-containing compounds.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- H3PO3 (Phosphorous acid)
- H3PO4 (Phosphoric acid)
- H4P2O7 (Pyrophosphoric acid)
- H3PO2 (Hypophosphorous acid)

2. **Draw the Structures**:
- **H3PO3**:
- Structure: P has one double bond with O and one single bond with OH. The remaining two H atoms are bonded to P.
- P-H bonds: 2
- **H3PO4**:
- Structure: P has one double bond with O and three single bonds with OH groups.
- P-H bonds: 0 (all H are part of OH groups, not P-H bonds)
- **H4P2O7**:
- Structure: This compound contains two phosphorus atoms. Each phosphorus can have multiple bonds with oxygen and OH groups.
- P-H bonds: 0 (typically, this compound does not contain P-H bonds)
- **H3PO2**:
- Structure: P has one double bond with O and one single bond with OH. The remaining two H atoms are bonded to P.
- P-H bonds: 2

3. **Count the P-H Bonds**:
- H3PO3: 2 P-H bonds
- H3PO4: 0 P-H bonds
- H4P2O7: 0 P-H bonds
- H3PO2: 2 P-H bonds

4. **Determine the Maximum**:
- The maximum number of reducing hydrogens is found in H3PO3 and H3PO2, both having 2 P-H bonds. However, H3PO3 is typically more recognized for its reducing properties.

5. **Conclusion**:
- The compound with the maximum number of reducing hydrogens is H3PO3 (Phosphorous acid).

### Final Answer:
- The maximum number of reducing hydrogens is contained in **H3PO3**.