Question

An acid $HA$ ionises as
$HA\rightleftharpoons H^{+}+A^{-}$
The pH of $1.0$M solution is $5$. Its dissociation constant would be:

Answer 1

an acid HA ionises as

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

given, $PH=5$

$\Rightarrow \left[H^{+}\right]={10}^{-5}=\left[A^{-}\right]$ $[\because$ Since, equal moles on dissociation]

also 1H solution means $\left[HA\right]=1$

now, $K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]}=\frac{\left({10}^{-5}\right)\left({10}^{-5}\right)}{1}={10}^{-10}$

$\therefore$ dissociation constant would be $K_a=1.0\times {10}^{-10}.$