First slide

Basic Buffer

Question

A weak base BOH is titrated with a strong acid HA. When 10 mL of HA is added, pH is found to be 9.00 and when 25 mL is added, pH is 8.00. The volume of the acid required to reach the equivalence point is

Difficult

A

55 mL
B

40 mL
C

30 mL
D

50 mL

Solution

$pH = 14 - {pK}_{b} - \log \frac{[B^{+}]}{[BOH]}$
V = volume of acid required for the equivalence point
$(i) ({pH}_{1}) = 9 = 14 - {pK}_{b} - \log \frac{10}{V - 10}$
$(ii) ({pH}_{2}) = 8 = 14 - {pK}_{b} - \log \frac{25}{V - 25}$
Eq. (i) - Eq. (ii)
$1 = \log \frac{25}{V - 25} - \log \frac{10}{V - 10} = \log \frac{25 (V - 10)}{10 (V - 25)}$
$\Rightarrow$ V = 30mL

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