pH of a salt solution of weak acid `(pK_(a) = 4)` & weak base
`(pK_(b) = 5)` at `25^(@)C` is :

Options

A. 6.5
B. 6
C. 7
D. 7.5

Explanation

To calculate the pH of a salt solution formed from a weak acid and a weak base, we can use the following formula:

\[ \text{pH} = \frac{1}{2} \text{pK}_w + \text{pK}_a - \text{pK}_b \]

### Step-by-step Solution:

1. **Identify the values given in the problem:**
- pK_a = 4 (for the weak acid)
- pK_b = 5 (for the weak base)
- pK_w = 14 (the ion product of water at 25°C)

2. **Substitute the values into the formula:**
\[ \text{pH} = \frac{1}{2} \times 14 + 4 - 5 \]

3. **Calculate \(\frac{1}{2} \times 14\):**
\[ \frac{1}{2} \times 14 = 7 \]

4. **Combine the values:**
\[ \text{pH} = 7 + 4 - 5 \]
\[ \text{pH} = 7 + (-1) = 6 \]

5. **Final calculation:**
\[ \text{pH} = 6 \]

### Conclusion:
The pH of the salt solution of the weak acid and weak base is **6**.