When a solution of formic acid was titrated with KOH solution, the pH of the solution was 3.65 when half the acid was neutralized. Calculate `K_a (HCOOH)`.

Explanation

To calculate the acid dissociation constant \( K_a \) for formic acid (HCOOH) when half of it has been neutralized by KOH, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Situation**:
- We are given that the pH of the solution is 3.65 when half of the formic acid has been neutralized.
- At this point, the concentration of the formic acid (HCOOH) is equal to the concentration of its conjugate base (HCOOK) formed from the neutralization.

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