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 condition would cause the percent ionization of a weak acid to increase, we can analyze the factors affecting the degree of ionization (α) of the weak acid. Here is a step-by-step solution:

### Step 1: Understand Percent Ionization
Percent ionization is defined as the ratio of the concentration of the ionized form of the acid to the initial concentration of the acid, multiplied by 100. For a weak acid (HA) that partially ionizes in solution:
\[ \text{Percent Ionization} = \left( \frac{[\text{A}^-]}{[\text{HA}]_0} \right) \times 100 \]
where \([\text{A}^-]\) is the concentration of the ionized form and \([\text{HA}]_0\) is the initial concentration of the acid.

### Step 2: Apply the Concept of Dilution
According to the law of Ostwald's dilution, the degree of ionization (α) of a weak acid is inversely proportional to the square root of its concentration (C):
\[ \alpha \propto \frac{1}{\sqrt{C}} \]
This means that as the concentration of the acid decreases (i.e., when it is diluted), the degree of ionization increases.

### Step 3: Analyze the Options
1. **Addition of a Strong Acid**: This would increase the concentration of H⁺ ions in the solution, shifting the equilibrium of the weak acid to the left (Le Chatelier's principle), thus decreasing the percent ionization.

2. **Addition of a Salt Containing its Conjugate Base**: This would also shift the equilibrium to the left, decreasing the percent ionization of the weak acid.

3. **Diluting with More Water**: This decreases the concentration of the weak acid (C), which, as established, increases the degree of ionization (α). Therefore, this option would increase the percent ionization.

4. **Percent Ionization as a Constant**: This statement is incorrect; percent ionization can change based on concentration and other factors.

### Step 4: Conclusion
The only condition that would cause the percent ionization of a weak acid to increase is **diluting with more water**. Therefore, the correct answer is option 3.

### Final Answer
**Diluting with more water increases the percent ionization of a weak acid.**

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