First slide

Acidic buffer

Question

If 50 ml of 0.2 M KOH is added to 40 ml of 0.5 M HCOOH, the pH of the resulting solution is $(K_{a} = 1.8 \times 10^{- 4})$

Moderate

A

3.4
B

7.5
C

5.6
D

3.75

Solution

$pH = - \log K_{a} + \log \frac{[Salt]}{[Acid]} [Salt] = \frac{0.2 \times 50}{1000} = 0.01; [Acid] = \frac{0.5 \times 40}{1000} = 0.02 pH = - \log (1.8 \times 10^{- 4}) + \log \frac{0.01}{0.02} pH = 4 - \log (1.8) + \log 0.5 pH = 4 - \log (1.8) - 0.301 pH = 3.4$

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