Species acting both as Bronsted acid and base is

Options

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

Answer

To determine which species can act both as a Brønsted acid and a Brønsted base, we need to understand the definitions of these terms:

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

Now, let's analyze the options provided in the question step by step.

### Step 1: Analyze HSO₄⁻
- **Structure**: HSO₄⁻ has one hydrogen atom attached to a sulfur atom, which is also bonded to four oxygen atoms (two of which are double-bonded).
- **As a Brønsted Acid**: HSO₄⁻ can donate its hydrogen ion (H⁺) to become SO₄²⁻.
- **As a Brønsted Base**: HSO₄⁻ can accept a hydrogen ion (H⁺) to become H₂SO₄.
- **Conclusion**: HSO₄⁻ can act as both an acid and a base.

### Step 2: Analyze H₂BO₃⁻
- **Structure**: H₂BO₃⁻ has two hydroxyl groups (OH) and a negative charge on one of the oxygen atoms.
- **As a Brønsted Acid**: It can donate one of its hydrogen ions (H⁺) to become HBO₃²⁻.
- **As a Brønsted Base**: It can accept a hydrogen ion (H⁺) to become H₃BO₃.
- **Conclusion**: H₂BO₃⁻ can also act as both an acid and a base.

### Step 3: Analyze NH₃
- **Structure**: NH₃ (ammonia) has three hydrogen atoms bonded to a nitrogen atom.
- **As a Brønsted Acid**: NH₃ cannot donate a hydrogen ion, as it does not have an acidic hydrogen to give away.
- **As a Brønsted Base**: NH₃ can accept a hydrogen ion (H⁺) to become NH₄⁺.
- **Conclusion**: NH₃ only acts as a base, not as an acid.

### Step 4: Analyze OH⁻
- **Structure**: OH⁻ (hydroxide ion) consists of one oxygen atom bonded to one hydrogen atom with a negative charge.
- **As a Brønsted Acid**: OH⁻ cannot donate a hydrogen ion, as it is already negatively charged and would become too negative if it lost H⁺.
- **As a Brønsted Base**: OH⁻ can accept a hydrogen ion (H⁺) to become H₂O.
- **Conclusion**: OH⁻ only acts as a base, not as an acid.

### Final Conclusion
The species that can act both as a Brønsted acid and a Brønsted base are:
- **HSO₄⁻**
- **H₂BO₃⁻**