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^(+)`

Answer

To determine which species is an acid and also a conjugate base of another acid, we will analyze each option step by step.

### Step 1: Understand the Definitions
- **Acid**: A substance that donates a proton (H⁺).
- **Conjugate Base**: The species that remains after an acid donates a proton.

### Step 2: Analyze Each Option

#### Option 1: HS⁻ (Hydrogen Sulfide Ion)
1. **Identify if it can act as an acid**:
- HS⁻ can donate a proton (H⁺) to become S²⁻.
2. **Identify if it can act as a conjugate base**:
- HS⁻ can accept a proton (H⁺) to form H₂S (hydrogen sulfide).
3. **Conclusion**: HS⁻ is an acid (donates H⁺) and also a conjugate base (accepts H⁺ from H₂SO₄).

#### Option 2: H₂SO₄ (Sulfuric Acid)
1. **Identify if it can act as an acid**:
- H₂SO₄ can donate a proton to become HSO₄⁻.
2. **Identify if it can act as a conjugate base**:
- H₂SO₄ cannot accept a proton because its structure is stable and its octet is complete.
3. **Conclusion**: H₂SO₄ is an acid but not a conjugate base.

#### Option 3: OH⁻ (Hydroxide Ion)
1. **Identify if it can act as an acid**:
- OH⁻ cannot donate a proton as it would become O²⁻, which is unstable.
2. **Identify if it can act as a conjugate base**:
- OH⁻ can accept a proton to form H₂O.
3. **Conclusion**: OH⁻ is a conjugate base but not an acid.

#### Option 4: H₃O⁺ (Hydronium Ion)
1. **Identify if it can act as an acid**:
- H₃O⁺ can donate a proton to become H₂O.
2. **Identify if it can act as a conjugate base**:
- H₃O⁺ cannot accept a proton as its structure is stable and its octet is complete.
3. **Conclusion**: H₃O⁺ is an acid but not a conjugate base.

### Final Conclusion
Based on the analysis, **Option 1 (HS⁻)** is the only species that acts as both an acid and a conjugate base of another acid.

### Summary of the Answer
- The species that is an acid and also a conjugate base of another acid is **HS⁻**.