First slide

Acidic buffer

Question

100 mL of 0.02 M benzoic acid $({pK}_{a} = 4.2)$ is titrated using 0.02 M NaOH. pH values after 50 mL and 100 mL of NaOH have been added are

Difficult

A

3.70, 7
B

4.2, 7
C

4.2, 8.1
D

4..2, 8.25

Solution

$On 50 ml NaOH addition C_{6} H_{5} COOH + {OH}^{-} \overset{}{\to} C_{6} H_{5} {COO}^{-} + H_{2} O t = 0 2 1 0 t_{eq} 1 1 1 pH = {pK}_{a} = 4.2 On 100 ml NaOH addition C_{6} H_{5} COOH + {OH}^{-} \overset{}{\to} C_{6} H_{5} {COO}^{-} + H_{2} O t = 0; 2 2 0 t_{eq} 2 2 2 [C_{6} H_{5} {COO}^{-}] = \frac{2}{200} = 0.01 M ∴ pH = 7 + {pK}_{a} + \frac{1}{2} \log C = 7 + \frac{4.2}{2} + \frac{1}{2} \log (0.01) = 8.1$

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