First slide

Ionization constants of weak acids, PH of weak acids

Question

The dissociation constant of a weak acid is $1.0 \times 10^{- 5},$ the equilibrium constant for the reaction with strong base is

Moderate

A

$1.0 \times 10^{- 5}$
B

$1.0 \times 10^{- 9}$
C

$1.0 \times 10^{9}$
D

$1.0 \times 10^{14}$

Solution

$HA ⇌ H^{+} + A^{-}; K_{a} = \frac{[H^{+}] [A^{-}]}{[HA]}$
Neutralization of the weak acid with strong base is
$HA + {OH}^{-} ⇌ A^{-} + H_{2} O$
$K = \frac{[A^{-}]}{[HA] [{OH}^{-}]}$
Dividing (i) by (ii), $\frac{K_{a}}{K} = [H^{+}] [{OH}^{-}] Given, K_{a} = 10^{- 5}$
$K = \frac{K_{a}}{K_{w}} = \frac{10^{- 5}}{10^{- 14}} = 10^{9}$

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