Question

The concentration of $H^{+}$ ions in $0.10M$ solution of a weak acid is $1.0\times {10}^{-5}mol{L}^{-1}$. Calculate the dissociation constant of the acid.

Answer 1

$HA\rightleftharpoons H^{+}{+A}^{-}$

Initial concentration $0.1$ $0$ $0$

Equilbrium concentration $0.1-1\times {10}^{-5}$$1\times {10}^{-5}$$1\times {10}^{-5}$

$\left[HA\right]$ can be taken as $0.1M$ as $1\times {10}^{-5}$ is very small.

$K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]}=\frac{1\times {10}^{-5}\times 1\times {10}^{-5}}{0.1}={10}^{-9}$