First slide

Ionization constants of weak bases, PH of weak bases

Question

Calculate pH of 0.10 N acetic acid having $K_{a} = 1.8 \times 10^{- 5}$

Moderate

A

2.8
B

5.6
C

6.8
D

3.4

Solution

${CH}_{3} COOH ⇌ {CH}_{3} {COO}^{-} + H^{+} (weak acid)$
Initially l 0 0
At equil. (1- $α$ ) $α$ $α$
$[H^{+}] = Cα or C \cdot \sqrt{\frac{K_{a}}{C}} = \sqrt{K_{a} C}$
$= \sqrt{1.8 \times 10^{- 5} \times 0.1} = 1.34 \times 10^{- 3} M$
$pH = - \log [H^{+}] = - \log (1.34 \times 10^{- 3}) = 2.87$

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