The equilibrium constant for the given reaction is approximately `10^(-3)`
`HPO_(4)^(2-)(aq) +HCO_(3)^(-) (aq)hArr H_(2)PO_(4)^(-) (aq) +CO_(3)^(2-)(aq)`
Which is strongest conjugate base in the given reaction?

Options

A. `HPO_(4)^(2-)(aq)`
B. `HCO_(3)^(-)(aq)`
C. `H_(2)PO_(4)^(-)(aq)`
D. `CO_(3)^(2-)(aq)`

Answer

To determine the strongest conjugate base in the given reaction, we need to analyze the species involved and their ability to accept protons. The reaction is:

\[ \text{HPO}_4^{2-} (aq) + \text{HCO}_3^{-} (aq) \rightleftharpoons \text{H}_2\text{PO}_4^{-} (aq) + \text{CO}_3^{2-} (aq) \]

### Step-by-Step Solution:

1. **Identify the Species**: The species involved in the reaction are:
- HPO₄²⁻ (dihydrogen phosphate)
- HCO₃⁻ (bicarbonate)
- H₂PO₄⁻ (hydrogen phosphate)
- CO₃²⁻ (carbonate)

2. **Understand Conjugate Acids and Bases**:
- A conjugate base is formed when an acid donates a proton (H⁺).
- Conversely, a conjugate acid is formed when a base accepts a proton.

3. **Analyze Each Species**:
- **HPO₄²⁻**:
- Can donate a proton to form H₂PO₄⁻ (acting as an acid).
- Can accept a proton to form PO₄³⁻ (acting as a base).
- Amphoteric in nature.
- **HCO₃⁻**:
- Can donate a proton to form CO₃²⁻ (acting as an acid).
- Can accept a proton to form H₂CO₃ (acting as a base).
- Amphoteric in nature.
- **H₂PO₄⁻**:
- Can donate a proton to form HPO₄²⁻ (acting as an acid).
- Can accept a proton to form H₃PO₄ (acting as a base).
- Amphoteric in nature.
- **CO₃²⁻**:
- Cannot donate a proton (no H⁺ to lose).
- Can accept a proton to form HCO₃⁻ (acting as a base).
- Acts only as a base.

4. **Determine the Strongest Conjugate Base**:
- Among the species, CO₃²⁻ can only accept a proton, making it a strong base.
- HPO₄²⁻, HCO₃⁻, and H₂PO₄⁻ can act as both acids and bases, but CO₃²⁻ is the only one that does not have a proton to donate, making it the strongest conjugate base.

5. **Conclusion**: Therefore, the strongest conjugate base in the given reaction is CO₃²⁻.

### Final Answer:
The strongest conjugate base in the given reaction is **CO₃²⁻**.

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