Species acting as both Bronsted acid and base is:

Options

A. `HSO_(4)^(-)`
B. `Na_(2)CO_(3)`
C. `NH_(3)`
D. `OH^(-)`

Explanation

To determine which species acts as both a Brønsted acid and a Brønsted base, we need to understand the definitions of Brønsted acids and bases:

1. **Brønsted Acid**: A substance that can donate a proton (H⁺ ion).
2. **Brønsted Base**: A substance that can accept a proton (H⁺ ion).

Now, let's analyze the given options:

1. **HSO₄⁻ (Hydrogen sulfate ion)**:
- As a Brønsted acid: HSO₄⁻ can donate a proton to form SO₄²⁻ (sulfate ion).
- As a Brønsted base: HSO₄⁻ can accept a proton to form H₂SO₄ (sulfuric acid).
- Conclusion: HSO₄⁻ can act as both a Brønsted acid and a Brønsted base.

2. **Na₂CO₃ (Sodium carbonate)**:
- Sodium carbonate primarily acts as a base because it can accept protons to form HCO₃⁻ (bicarbonate ion), but it does not donate protons effectively.
- Conclusion: It does not act as a Brønsted acid.

3. **NH₃ (Ammonia)**:
- Ammonia acts primarily as a Brønsted base because it can accept a proton to form NH₄⁺ (ammonium ion). However, it does not donate protons effectively.
- Conclusion: It does not act as a Brønsted acid.

4. **OH⁻ (Hydroxide ion)**:
- Hydroxide ion acts as a Brønsted base because it can accept a proton to form H₂O (water). However, it does not donate protons.
- Conclusion: It does not act as a Brønsted acid.

Based on this analysis, the only species that can act as both a Brønsted acid and a Brønsted base is **HSO₄⁻**.

**Final Answer**: HSO₄⁻