Question

A $0.1M$ solution of weak acid $HA$ is $1$% dissociation at ${25}^{o}C$. What is its $K_a$?

• A. ${10}^{-5}$
• B. ${10}^{-4}$
• C. ${10}^{-3}$
• D. ${10}^{-8}$

Answer 1

The correct option is A ${10}^{-5}$

For weak acid $HA$:
${\alpha }_{HA}=\frac{1}{100}=0.01$

$\left[HA\right]=0.1M$

$\therefore$ ${Ka}_{\alpha }=c{\alpha }^2=0.1\times {0.01}^2={10}^{-5}$