Which one of the following species acts as both Bronsted acid and base ?

Options

A. `H_2PO_(2)^(-)`
B. `H_2PO_(3)^(2-)`
C. `H_2PO_(4)^(2-)`
D. All of these

Answer

To determine which species acts as both a Bronsted acid and a Bronsted base, we need to analyze each option based on the definitions of Bronsted acids and bases.

**Step 1: Understand the Definitions**
- A Bronsted acid is a species that can donate a proton (H⁺ ion).
- A Bronsted base is a species that can accept a proton (H⁺ ion).

**Step 2: Analyze Each Species**
1. **H₂PO₂⁻ (Dihydrogen phosphite ion)**
- As a Bronsted acid: It can donate a proton to form HPO₂²⁻.
- As a Bronsted base: It can accept a proton to form H₃PO₂.
- Conclusion: H₂PO₂⁻ acts as both a Bronsted acid and a Bronsted base.

2. **H₂PO₃²⁻ (Dihydrogen phosphonate ion)**
- As a Bronsted acid: It can donate a proton to form HPO₃³⁻.
- As a Bronsted base: It can accept a proton to form H₃PO₃⁻.
- Conclusion: H₂PO₃²⁻ acts as both a Bronsted acid and a Bronsted base.

3. **H₂PO₄²⁻ (Dihydrogen phosphate ion)**
- As a Bronsted acid: It can donate a proton to form HPO₄³⁻.
- As a Bronsted base: It can accept a proton to form H₃PO₄⁻.
- Conclusion: H₂PO₄²⁻ acts as both a Bronsted acid and a Bronsted base.

**Step 3: Final Conclusion**
Since all three species (H₂PO₂⁻, H₂PO₃²⁻, and H₂PO₄²⁻) can act as both Bronsted acids and Bronsted bases, the answer is:
- **All of these (Option 4)**.