Assertion : For acidic buffer, if ratio of conjugate acid- base pair is changed by 10 times, pH change by `pm` 1.
Reason : pH always increases or decreases by 1 unit.

Options

A. Assertion and reason both are correct statements and Reason is correct explanation for assertion.
B. Assertion and reason both are correct statement but reason is not correct explanation for assertion
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement

Answer

To solve the question regarding the assertion and reason about the pH change in an acidic buffer when the ratio of the conjugate acid-base pair is changed, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Buffer Equation**:
The pH of an acidic buffer solution is given by the Henderson-Hasselbalch equation:
\[ \text{pH} = \text{pKa} + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \]
where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the conjugate acid.

2. **Initial Condition**:
Let’s assume the initial ratio of the conjugate acid-base pair is \(\frac{[\text{A}^-]}{[\text{HA}]} = x\). Thus, the initial pH can be expressed as:
\[ \text{pH}_1 = \text{pKa} + \log(x) \]

3. **Changing the Ratio**:
If the ratio is changed by 10 times, we can express the new ratio as:
\[ \frac{[\text{A}^-]}{[\text{HA}]} = 10x \]
Now, the new pH can be calculated as:
\[ \text{pH}_2 = \text{pKa} + \log(10x) \]

4. **Calculating the New pH**:
Using the logarithmic property \(\log(ab) = \log(a) + \log(b)\), we can write:
\[ \text{pH}_2 = \text{pKa} + \log(10) + \log(x) \]
Since \(\log(10) = 1\), we have:
\[ \text{pH}_2 = \text{pKa} + 1 + \log(x) \]

5. **Finding the Change in pH**:
Now, we can find the change in pH:
\[ \text{pH}_2 - \text{pH}_1 = (\text{pKa} + 1 + \log(x)) - (\text{pKa} + \log(x)) = 1 \]
This shows that the pH changes by 1 unit when the ratio of the conjugate acid-base pair is changed by 10 times.

6. **Evaluating the Assertion and Reason**:
- **Assertion**: "For acidic buffer, if the ratio of conjugate acid-base pair is changed by 10 times, pH changes by ±1." This is **true**.
- **Reason**: "pH always increases or decreases by 1 unit." This is **false** because the change in pH depends on the factor by which the ratio is changed (e.g., changing the ratio by 100 would change the pH by 2).

### Final Conclusion:
- The assertion is correct, while the reason is incorrect. Therefore, the answer to the question is that the assertion is true, but the reason is false.