A certain volume of a monobasic weak acid was titrated against `0.1` M NaOH solution . The end point reached upon addition of 30 ml of alkali . The pH of the solution upon the addition of 20 ml of alkali was` 6.3` . Find `K_(a)` for the weak acid .

Explanation

To solve the problem, we need to find the dissociation constant \( K_a \) for a monobasic weak acid (HA) that was titrated with a 0.1 M NaOH solution. The key information provided includes that the endpoint of the titration was reached after adding 30 mL of NaOH, and the pH of the solution after adding 20 mL of NaOH was 6.3.

### Step-by-Step Solution:

1. **Calculate moles of NaOH at the endpoint:**
\[ \text{Moles of NaOH} = \text{Molarity} \times \text{Volume} = 0.1 \, \text{mol/L} \times 30 \, \text{mL} = 0.1 \, \text{mol/L} \times 0.030 \, \text{L} = 0.003 \, \text{mol} \]
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