Among the following, which causes the greatest change in pH on addition to 50 ml of 0.2 M oxalic acid solution?

Options

A. Addition of 25 ml of 0.02 M oxalic acid
B. Addition of 25 ml of 1 M NaOH solution
C. Addition of 2 ml of 0.02 M `NH_(4)OH` solution
D. Addition of 50 ml of 0.2 M acetic acid solution

Answer

To determine which addition causes the greatest change in pH when added to 50 ml of 0.2 M oxalic acid solution, we will analyze each option step by step.

### Step 1: Understand the Properties of Oxalic Acid
Oxalic acid (H₂C₂O₄) is a weak acid. When dissolved in water, it partially dissociates to produce hydrogen ions (H⁺). The dissociation can be represented as:
\[ \text{H}_2\text{C}_2\text{O}_4 \rightleftharpoons \text{H}^+ + \text{HC}_2\text{O}_4^- \]

### Step 2: Analyze Each Option
1. **Addition of 25 ml of 0.02 M Oxalic Acid**:
- This will add more H⁺ ions to the solution, thus decreasing the pH. However, since it is still an acid, it will not cause a significant change in pH compared to the original solution.

2. **Addition of 25 ml of 1 M NaOH**:
- NaOH is a strong base. When added to the oxalic acid solution, it will react with the H⁺ ions, neutralizing them and forming water and a basic salt. This will significantly increase the pH, as the solution will become more basic.

3. **Addition of 2 ml of 0.02 M NH₄OH**:
- NH₄OH (ammonium hydroxide) is a weak base. Adding this to the weak acid will result in a slight increase in pH, but not as much as a strong base would. The change will be minimal.

4. **Addition of 50 ml of 0.2 M Acetic Acid**:
- Acetic acid is also a weak acid. Adding it will increase the concentration of H⁺ ions and decrease the pH. Similar to the first option, this will not cause a significant change in pH.

### Step 3: Compare the Effects on pH
- The addition of weak acids (options 1 and 4) will result in a decrease in pH.
- The addition of a weak base (option 3) will result in a slight increase in pH.
- The addition of a strong base (option 2) will result in a significant increase in pH due to the neutralization of H⁺ ions and the formation of a basic salt.

### Conclusion
The addition of **25 ml of 1 M NaOH** will cause the greatest change in pH when added to the 50 ml of 0.2 M oxalic acid solution.

### Final Answer
**The correct option is: Addition of 25 ml of 1 M NaOH solution.**

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