Which of the following behave both as Bronsted acids as well Bronsted base?

Options

A. `H_2O`
B. `H_3PO_4`
C. `HCO_3^-`
D. HCOOH

Answer

To determine which compounds can behave both as Bronsted acids and Bronsted bases, we need to understand the definitions of Bronsted acids and bases:

1. **Bronsted Acid**: A substance that donates a proton (H⁺ ion).
2. **Bronsted Base**: A substance that accepts a proton (H⁺ ion).

Now, let's analyze the given options step by step.

### Step 1: Analyze H2O (Water)
- **As a Bronsted Acid**: Water can donate a proton (H⁺) to form OH⁻ (hydroxide ion).
\[ \text{H}_2\text{O} \rightarrow \text{H}^+ + \text{OH}^- \]
- **As a Bronsted Base**: Water can accept a proton (H⁺) to form H₃O⁺ (hydronium ion).
\[ \text{H}_2\text{O} + \text{H}^+ \rightarrow \text{H}_3\text{O}^+ \]
- **Conclusion**: H₂O can act as both a Bronsted acid and a Bronsted base.

### Step 2: Analyze H₃PO₄ (Phosphoric Acid)
- **As a Bronsted Acid**: H₃PO₄ can donate a proton to form H₂PO₄⁻.
\[ \text{H}_3\text{PO}_4 \rightarrow \text{H}^+ + \text{H}_2\text{PO}_4^- \]
- **As a Bronsted Base**: H₃PO₄ does not accept protons in typical reactions.
- **Conclusion**: H₃PO₄ primarily acts as a Bronsted acid, not a base.

### Step 3: Analyze HCO₃⁻ (Bicarbonate Ion)
- **As a Bronsted Acid**: HCO₃⁻ can donate a proton to form CO₃²⁻ (carbonate ion).
\[ \text{HCO}_3^- \rightarrow \text{H}^+ + \text{CO}_3^{2-} \]
- **As a Bronsted Base**: HCO₃⁻ can accept a proton to form H₂CO₃ (carbonic acid).
\[ \text{HCO}_3^- + \text{H}^+ \rightarrow \text{H}_2\text{CO}_3 \]
- **Conclusion**: HCO₃⁻ can act as both a Bronsted acid and a Bronsted base.

### Final Conclusion
Based on the analysis:
- **H₂O** and **HCO₃⁻** behave as both Bronsted acids and Bronsted bases.
- **H₃PO₄** behaves only as a Bronsted acid.

### Answer
The compounds that behave both as Bronsted acids and Bronsted bases are **H₂O** and **HCO₃⁻**.