Question

A weak acid reacts with strong base, ionisation constant of weak acid is ${10}^{-4}$. Find out equilibrium constant for this reaction.

• A. ${10}^{-10}$
• B. ${10}^{10}$
• C. ${10}^{-9}$
• D. ${10}^9$

Answer 1

Answer: (B)

${10}^{10}$

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

ionic product $K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]}$

Given that,

$K_a={10}^{-4}$

Strong Base $BOH$ reaction

$HA+O{H}^{-}\rightleftharpoons H_{2}O+A^{-}$

$K=\frac{\left[A^{-}\right]\left[H_{2}O\right]}{\left[{OH}^{-}\right]\left[HA\right]}$

$K_w=\left[H^{+}\right]\left[O{H}^{-}\right]={10}^{-14}$

$K=\frac{K_a}{K_w}=\frac{{10}^{-4}}{{10}^{-14}}={10}^{10}$

Hence, option $B$ is correct.