Which of the following species can be formed in acid-base reactions as Bronsted base as well as Bronsted acids?

Options

A. `[Fe(H_(2)O)_(5)]^(3+)`
B. `HPO_(4)^(-)`
C. `SO_(4)^(2)`
D. `NH_(4)^(+)`

Answer

To determine which species can act as both a Brønsted acid and a Brønsted base, we need to analyze each option provided in the question. A Brønsted acid is a species that donates a proton (H⁺), while a Brønsted base is a species that accepts a proton.

### Step-by-Step Solution:

1. **Understanding Brønsted Acids and Bases**:
- A Brønsted acid donates a proton (H⁺).
- A Brønsted base accepts a proton (H⁺).

2. **Analyzing Each Option**:
- **Option 1: Fe(H₂O)₅³⁺**:
- This complex can donate a proton to form Fe(H₂O)₄(OH)²⁺, indicating it can act as a Brønsted acid.
- However, it cannot accept a proton because it lacks available orbitals to accommodate an additional H⁺ ion.
- **Conclusion**: It is only a Brønsted acid.

- **Option 2: HPO₄²⁻**:
- This species can accept a proton to form H₂PO₄⁻ (acting as a Brønsted base).
- It can also donate a proton to form PO₄³⁻ (acting as a Brønsted acid).
- **Conclusion**: It can act as both a Brønsted acid and a Brønsted base.

- **Option 3: SO₄²⁻**:
- This species can accept a proton to form HSO₄⁻ (acting as a Brønsted base).
- However, it does not have a proton to donate, as it does not contain any hydrogen atoms that can be released.
- **Conclusion**: It is only a Brønsted base.

- **Option 4: NH₄⁺**:
- This species can donate a proton to form NH₃ (acting as a Brønsted acid).
- However, it cannot accept a proton because it lacks empty orbitals to accommodate an additional H⁺ ion.
- **Conclusion**: It is only a Brønsted acid.

3. **Final Conclusion**:
- The only species that can act as both a Brønsted acid and a Brønsted base is **Option 2: HPO₄²⁻**.

### Answer:
**Option 2: HPO₄²⁻ can act as both a Brønsted acid and a Brønsted base.**