First slide

Ionization constants of weak acids, PH of weak acids

Question

A weak acid (HA) after treatment with 12 mL of 0.1 M strong base (BOH) has a pH of 5. At the end point, the volume of same base required is 27 mL. $K_{a}$ of acid is (log 2 = 0.3)

Moderate

A

$1.8 \times 10^{- 5}$
B

$8 \times 10^{- 6}$
C

$1.8 \times 10^{- 6}$
D

$8 \times 10^{- 5}$

Solution

mmoles of HA taken = 27 x 0.1 = 2.7
$HA + {OH}^{-} \overset{}{\to} A^{-} + H_{2} O t = 0 2.7 1.2 t_{eq} 1.5 - 1.2 pH = {pK}_{a} + \log \frac{[A^{-}]}{[HA]} \Rightarrow 5 = {pK}_{a} + \log (\frac{1.2}{1.5}) 5 = {pK}_{a} + \log \frac{4}{5} ∴ {pK}_{a} = 5.1 \Rightarrow K_{a} = 8 \times 10^{- 6} .$

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