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 (H⁺ ions).

### Step-by-Step Solution:

1. **Identify the Species Involved**:
The reaction involves the following species:
- 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⁺).
- The strength of a conjugate base is inversely related to the strength of its corresponding acid. A weaker acid will have a stronger conjugate base.

3. **Analyze Each Species**:
- **HPO₄²⁻**:
- Can lose H⁺ to form H₂PO₄⁻ (acid) or accept H⁺ to form PO₄³⁻ (base).
- Amphoteric (can act as both acid and base).
- **HCO₃⁻**:
- Can lose H⁺ to form CO₃²⁻ (base) or accept H⁺ to form H₂CO₃ (acid).
- Amphoteric.
- **H₂PO₄⁻**:
- Can lose H⁺ to form HPO₄²⁻ (base) or accept H⁺ to form H₃PO₄ (acid).
- Amphoteric.
- **CO₃²⁻**:
- Can only accept H⁺ to form HCO₃⁻ (acid).
- Acts as a base.

4. **Determine the Strength of Conjugate Bases**:
- Among the species, CO₃²⁻ does not have any hydrogen atoms and can only act as a base by accepting a proton.
- The other species (HPO₄²⁻, HCO₃⁻, H₂PO₄⁻) can act as both acids and bases, but they are less basic than CO₃²⁻.

5. **Conclusion**:
- Since CO₃²⁻ is the only species that can only act as a base and does not have any acidic protons, it is the strongest conjugate base in this reaction.

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

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