Which of the following species is an acid and also a conjugate base of another acid

Options

A. `HSO_(4)^(-)`
B. `H_(2)SO_(4)`
C. `OH^(-)`
D. `H_(3)O^(+)`

Explanation

To solve the question of which species is an acid and also a conjugate base of another acid, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understanding Acids and Bases**:
- An acid is defined as a species that donates protons (H⁺ ions).
- A base is defined as a species that accepts protons (H⁺ ions).
- The conjugate base of an acid is formed when the acid donates a proton. Conversely, the conjugate acid of a base is formed when the base accepts a proton.

2. **Analyzing Each Option**:
- **Option A: HSO₄⁻**
- HSO₄⁻ can donate a proton to form SO₄²⁻ (its conjugate base).
- HSO₄⁻ can also accept a proton to form H₂SO₄ (its conjugate acid).
- Therefore, HSO₄⁻ is both an acid (when it acts as a proton donor) and a conjugate base (when it forms SO₄²⁻).

- **Option B: H₂SO₄**
- H₂SO₄ can donate a proton to form HSO₄⁻ (its conjugate base).
- However, H₂SO₄ cannot act as a conjugate base for another acid because it cannot accept a proton to form H₃SO₄⁺ (this would lead to an impossible oxidation state for sulfur).
- Therefore, H₂SO₄ does not meet the criteria.

- **Option C: OH⁻**
- OH⁻ can donate a proton to form O²⁻ (its conjugate base).
- OH⁻ can also accept a proton to form H₂O (its conjugate acid).
- Thus, OH⁻ is both an acid (when it donates a proton) and a conjugate base (when it forms O²⁻).

- **Option D: H₃O⁺**
- H₃O⁺ can donate a proton to form H₂O (its conjugate base).
- However, H₃O⁺ cannot act as a conjugate base for another acid because it cannot accept a proton to form H₄O²⁺ (which is not feasible).
- Therefore, H₃O⁺ does not meet the criteria.

3. **Conclusion**:
- The species that is an acid and also a conjugate base of another acid is **HSO₄⁻** and **OH⁻**.

### Final Answer:
The correct options are **HSO₄⁻** and **OH⁻**.

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