In the equilibrium,
`H_3PO_4 + H_2O hArr H_2PO_4^(-) + H_3O^(+)`
the equilibrium constant is most likely to be changed by:

Options

A. adding `H_2PO_4^-` ion
B. adding `H_3PO_4`
C. adding a catalyst
D. heating the mixture

Answer

To determine how the equilibrium constant for the reaction

\[ H_3PO_4 + H_2O \rightleftharpoons H_2PO_4^- + H_3O^+ \]

can be changed, we need to analyze the factors that affect the equilibrium constant (K).

### Step-by-Step Solution:

1. **Understanding the Equilibrium Constant (K):**
The equilibrium constant for the reaction is given by the expression:
\[ K = \frac{[H_2PO_4^-][H_3O^+]}{[H_3PO_4][H_2O]} \]
Here, the concentrations of the products are in the numerator and the concentrations of the reactants are in the denominator.

2. **Factors Affecting K:**
The equilibrium constant (K) is affected primarily by temperature. Changes in concentration or pressure do not change the value of K; they only shift the position of equilibrium.

3. **Effect of Temperature:**
- If the reaction is exothermic (releases heat), increasing the temperature will shift the equilibrium to the left (towards the reactants) and decrease K.
- If the reaction is endothermic (absorbs heat), increasing the temperature will shift the equilibrium to the right (towards the products) and increase K.

4. **Analyzing the Given Options:**
- **Adding H3PO4:** This will not change K; it only shifts the equilibrium position.
- **Adding H2PO4^-:** This also will not change K; it shifts the equilibrium position.
- **Adding a Catalyst:** Catalysts speed up the rate of reaching equilibrium but do not change the equilibrium constant.
- **Heating the Mixture:** This can change K depending on whether the reaction is exothermic or endothermic.

5. **Conclusion:**
The equilibrium constant is most likely to be changed by heating the mixture. Therefore, the answer is that the equilibrium constant is affected by temperature changes.

### Final Answer:
The equilibrium constant is most likely to be changed by heating the mixture.

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