pKa of waek acid is 3.0. Effective range of a buffer of HA and `A^(-)` is about pH :-

Options

A. 3 to 4
B. 3 to 6
C. 2 to 3
D. 2 to 4

Answer

To find the effective range of a buffer solution made from a weak acid (HA) and its conjugate base (A⁻), we can use the Henderson-Hasselbalch equation. The effective pH range of a buffer is typically considered to be within one pH unit above and below the pKa of the weak acid.

### Step-by-Step Solution:

1. **Identify the given pKa value**:
- The problem states that the pKa of the weak acid (HA) is 3.0.

2. **Determine the effective range of the buffer**:
- The effective range of a buffer is calculated using the formula:
\[ \text{Effective range} = \text{pKa} - 1 \quad \text{to} \quad \text{pKa} + 1 \]
- Substitute the given pKa value into the formula:
\[ \text{Effective range} = 3.0 - 1 \quad \text{to} \quad 3.0 + 1 \]

3. **Calculate the lower limit**:
- Calculate the lower limit:
\[ 3.0 - 1 = 2.0 \]

4. **Calculate the upper limit**:
- Calculate the upper limit:
\[ 3.0 + 1 = 4.0 \]

5. **State the effective pH range**:
- Therefore, the effective pH range of the buffer solution is:
\[ \text{pH} = 2.0 \quad \text{to} \quad 4.0 \]

6. **Select the correct option**:
- Based on the calculated range, the correct answer corresponds to the range of pH values from 2.0 to 4.0.

### Final Answer:
The effective range of a buffer of HA and A⁻ is about pH 2.0 to 4.0.

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