First slide

Acidic buffer

Question

20 mL of a weak monobasic acid (HA) requires 20 mL 0.2 M NaOH for complete titration. If pH of solution upon addition of 10 mL of this alkali to 25 mL of the above solution of HA is 5.8. The ${pK}_{a}$ of the weak acid is

Difficult

A

6.1
B

5.8
C

5.98
D

5.58

Solution

meq. of acid = meq of base $\Rightarrow 20 \times M = 20 \times 0.2 = 4$
Molarity of HA = 0.2 M
When 25ml,0.2M acid reacts with 10ml 0.2M NaOH
$HA + {OH}^{-} \overset{}{\to} A^{-} + H_{2} O 5 (= 25 \times . 2) 2 (10 \times . 2) 0 3 (= 5 - 2) - 2 mmole ∴ pH = {pK}_{a} + \log \frac{[A^{-}]}{[HA]} \Rightarrow 5.8 = {pK}_{a} + \log (\frac{2}{3}) \Rightarrow {pK}_{a} = 5.98$

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