Which of the following would cause the percent ionization of week acid to increase ?

Options

A. Addition of a strong acid
B. Addition of a salf containing its conjugate base
C. Diluting with more water
D. The percent ionization of a week acid is a constant and cannot be increased .

Answer

To determine which factor would cause the percent ionization of a weak acid to increase, we need to analyze the effects of different conditions on the ionization of weak acids. Let's break down the reasoning step by step:

### Step 1: Understanding Percent Ionization
Percent ionization is defined as the ratio of the concentration of ionized acid to the initial concentration of the acid, multiplied by 100. For weak acids, this value is typically less than 100% because they do not completely dissociate in solution.

### Step 2: Analyzing the Options
We have several options to consider:

1. **Addition of a Strong Acid**:
- When a strong acid is added to a weak acid, the concentration of H⁺ ions in the solution increases. This leads to the common ion effect, which suppresses the ionization of the weak acid. Therefore, the percent ionization of the weak acid **decreases**.

2. **Addition of a Salt Containing its Conjugate Base**:
- Similar to the addition of a strong acid, adding a salt that contains the conjugate base of the weak acid also introduces a common ion (the conjugate base). This again results in a decrease in the ionization of the weak acid due to the common ion effect. Thus, the percent ionization **decreases**.

3. **Diluting with More Water**:
- When a weak acid is diluted with water, the concentration of the acid decreases. According to Ostwald's dilution law, the degree of ionization (α) increases as the concentration (C) decreases. This means that the percent ionization of the weak acid **increases**.

4. **The Assertion that Percent Ionization Cannot be Increased**:
- This statement is incorrect because we have established that dilution with water can indeed increase the percent ionization of a weak acid.

### Conclusion
Based on the analysis of the options, the only condition that causes the percent ionization of a weak acid to increase is **diluting with more water**. Therefore, the correct answer is:

**C. Diluting with more water.**