First slide

Acidic buffer

Question

pH of a mixture of 1 M benzoic acid $({pK}_{a} = 4.20)$ and $1 M C_{6} H_{5} COONa$ is 4.5. In 300 ml buffer, benzoic acid is [log 2 = 0.3]

Moderate

A

200 ml
B

150 ml
C

100 ml
D

50 ml

Solution

$pH = {pK}_{a} + \log \frac{[Salt]}{[Acid]} ∴ 4.5 = 4.2 + \log \frac{[Salt]}{[Acid]} = \log \frac{[Salt]}{[Acid]} = 0.3 (since anti \log 0.3 = 2) ∴ \frac{[Salt]}{[Acid]} = 2$
Let V ml of 1 M $C_{6} H_{5}$ COOH solution and (300 - V) ml of 1 M $C_{6} H_{5} C O O N a$ solution be mixed together
$[Acid] = \frac{V \times 1}{1000} \times \frac{1000}{300}; [Salt] = \frac{(300 - V)}{1000} \times \frac{1000}{300} [Acid] = \frac{V}{300}; [Salt] = \frac{300 - V}{300} ∴ \frac{300 - V / 300}{V / 300} = 2 = 300 - V = 2 V ∴ V = 100 ml$

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