First slide

Ionization constants of weak acids, PH of weak acids

Question

An acid HA ionizes as $HA ⇌ H^{+} + A^{-}$ The pH of 1 M solution is 5. Its dissociation constant would be

Easy

A

$1 \times 10^{- 10}$
B

5
C

$5 \times 10^{- 8}$
D

$1 \times 10^{- 5}$

Solution

pH of a solution = 5
$∴ [H^{+}] = 10^{- pH} = 10^{- 5}$
HA (a weak acid) ionizes as
$HA ⇌ H^{+} + A^{-}$
1.0 0 0
$(1 - 10^{- 5})$ $10^{- 5}$ $10^{- 5}$
$K_{a} = \frac{[H^{-}] [A^{-}]}{[HA]} = \frac{10^{- 5} \times 10^{- 5}}{(1 - 10^{- 5})} = \frac{10^{- 10}}{1} [∵ 1 > > > 10^{- 5}] = 1 \times 10^{- 10} M$

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