Which of the following species acts as Bronsted base but not as acid ?

Options

A. `CH_(3)COOH `
B. `HCO_(3)^(-)`
C. `H_(2)PO_(2)`
D. `H_(2)PO_(3)^(-)`

Answer

To determine which species acts as a Brønsted base but not as an acid, we need to analyze each option provided in the question. A Brønsted base is defined as a substance that accepts protons (H⁺ ions), while a Brønsted acid is a substance that donates protons.

### Step-by-Step Solution:

1. **Identify the Options**:
Let's consider the given species:
- CH₃COOH (acetic acid)
- HCO₃⁻ (bicarbonate ion)
- H₂PO₂ (dihydrogen phosphite)
- H₂PO₃ (dihydrogen phosphite)

2. **Analyze CH₃COOH**:
- Reaction: CH₃COOH ⇌ CH₃COO⁻ + H⁺
- In this reaction, CH₃COOH donates a proton (H⁺) to form CH₃COO⁻.
- Conclusion: CH₃COOH acts as a Brønsted acid. **(Not a base)**

3. **Analyze HCO₃⁻**:
- Reaction: HCO₃⁻ ⇌ CO₃²⁻ + H⁺
- Here, HCO₃⁻ donates a proton to form CO₃²⁻.
- Conclusion: HCO₃⁻ acts as a Brønsted acid. **(Not a base)**

4. **Analyze H₂PO₂**:
- Reaction: H₂PO₂ + H⁺ ⇌ H₃PO₂
- In this case, H₂PO₂ accepts a proton to form H₃PO₂.
- Conclusion: H₂PO₂ acts as a Brønsted base. **(Not an acid)**

5. **Analyze H₂PO₃**:
- Reaction: H₂PO₃ ⇌ HPO₃²⁻ + H⁺
- Here, H₂PO₃ donates a proton to form HPO₃²⁻.
- Conclusion: H₂PO₃ acts as a Brønsted acid. **(Not a base)**

6. **Final Conclusion**:
- The only species that acts as a Brønsted base but not as an acid is **H₂PO₂**.

### Final Answer:
**H₂PO₂ acts as a Brønsted base but not as an acid.**