A monobasic acid of phosphorus , which reduces `HgCl_2` to balck Hg is

Options

A. hypophosphorus acid
B. phosphoric acid
C. metaphosphoric acid
D. pyrophosphoric acid

Answer

To solve the question regarding the monobasic acid of phosphorus that reduces HgCl₂ to black Hg, we can follow these steps:

### Step 1: Understanding Monobasic Acids
A monobasic acid is one that can donate only one proton (H⁺) per molecule in an acid-base reaction. This means it has only one hydroxyl (OH) group.

### Step 2: Identifying Phosphorus Acids
The common acids of phosphorus include:
- Hypophosphorous acid (H₃PO₂)
- Phosphoric acid (H₃PO₄)
- Metaphosphoric acid (HPO₃)
- Pyrophosphoric acid (H₄P₂O₇)

### Step 3: Analyzing Each Acid
1. **Hypophosphorous acid (H₃PO₂)**:
- Structure: H₃PO₂ has one OH group, making it a monobasic acid.
- Reducing property: It can reduce HgCl₂ to Hg.

2. **Phosphoric acid (H₃PO₄)**:
- Structure: H₃PO₄ has three OH groups, making it a tribasic acid.

3. **Metaphosphoric acid (HPO₃)**:
- Structure: HPO₃ can have multiple OH groups depending on its polymerization, thus not monobasic.

4. **Pyrophosphoric acid (H₄P₂O₇)**:
- Structure: H₄P₂O₇ has four OH groups, making it a tetrabasic acid.

### Step 4: Conclusion
The only monobasic acid of phosphorus that can reduce HgCl₂ to black Hg is **Hypophosphorous acid (H₃PO₂)**.

### Final Answer
The monobasic acid of phosphorus which reduces HgCl₂ to black Hg is **Hypophosphorous acid (H₃PO₂)**.

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