Statement: The pH of an aqueous solution of acetic acid remains unchanged on the addition of sodium acetate.
Explanation: The ionisation of acetic acid is suppressed by the addition of sodium acetate.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question regarding the statement and explanation about the pH of an aqueous solution of acetic acid and the addition of sodium acetate, we can break it down into a step-by-step analysis.

### Step 1: Understand the Assertion
The assertion states that "the pH of an aqueous solution of acetic acid remains unchanged on the addition of sodium acetate."

### Step 2: Analyze the Chemistry Involved
Acetic acid (CH3COOH) is a weak acid that partially ionizes in water:
\[ \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \]

When sodium acetate (CH3COONa) is added to this solution, it dissociates completely into its ions:
\[ \text{CH}_3\text{COONa} \rightarrow \text{CH}_3\text{COO}^- + \text{Na}^+ \]

### Step 3: Identify the Common Ion Effect
The acetate ion (CH3COO^-) from sodium acetate is a common ion with the acetate ion produced from the ionization of acetic acid. The presence of this common ion suppresses the ionization of acetic acid due to the common ion effect.

### Step 4: Determine the Effect on pH
When the ionization of acetic acid is suppressed, the concentration of hydrogen ions (H^+) in the solution decreases. Since pH is inversely related to the concentration of hydrogen ions, a decrease in H^+ concentration results in an increase in pH.

### Step 5: Conclusion on the Assertion
Since the assertion claims that the pH remains unchanged, but we have established that the pH actually increases upon the addition of sodium acetate, the assertion is incorrect.

### Step 6: Analyze the Explanation
The explanation states that "the ionization of acetic acid is suppressed by the addition of sodium acetate." This statement is true as we have established that the common ion effect leads to decreased ionization of acetic acid.

### Step 7: Final Evaluation
- The assertion is **false** (pH does change).
- The explanation is **true** (ionization is suppressed).

Thus, the correct conclusion is that the assertion is false while the reason is true.

### Final Answer
The correct answer is that the assertion is false and the reason is true.

---