Dissociation constant of a weak acid is decreased by

Options

A. Addition of a strong acid
B. Addition of a salt of the above weak acid
C. Decreasing temperature
D. Dilution of the solution

Explanation

To determine how the dissociation constant (Ka) of a weak acid is affected, let's analyze the factors that can lead to a decrease in Ka.

### Step-by-Step Solution:

1. **Understanding Dissociation of a Weak Acid**:
A weak acid (HA) dissociates in water to form hydrogen ions (H⁺) and its conjugate base (A⁻):
\[ HA \rightleftharpoons H^+ + A^- \]

2. **Expression for Dissociation Constant (Ka)**:
The dissociation constant (Ka) is given by the expression:
\[ K_a = \frac{[H^+][A^-]}{[HA]} \]
where [H⁺], [A⁻], and [HA] are the equilibrium concentrations of the ions and the undissociated acid.

3. **Factors Affecting Ka**:
To decrease Ka, we need to manipulate the equilibrium. Since the concentration of the acid (C) is fixed, we need to focus on the degree of dissociation (α).

4. **Effect of Temperature**:
- **Increasing Temperature**: Generally, for weak acids, increasing the temperature leads to an increase in the degree of dissociation (α), which in turn increases Ka.
- **Decreasing Temperature**: Conversely, decreasing the temperature leads to a decrease in α, which results in a decrease in Ka.

5. **Conclusion**:
Therefore, to decrease the dissociation constant (Ka) of a weak acid, we should **decrease the temperature**.

6. **Other Factors**:
- **Addition of Strong Acid**: Adding a strong acid increases the concentration of H⁺ ions, which shifts the equilibrium to the left, increasing Ka.
- **Addition of Salt of Weak Acid**: This also increases the concentration of the conjugate base, which can shift the equilibrium and increase Ka.

### Final Answer:
The dissociation constant (Ka) of a weak acid is decreased by **decreasing the temperature**.