Question

The pH of 0.1 M solution of weak acid is 3. What is the value of ionisation constant for the acid

• A. 0.1
• B. ${10}^{-3}$
• C. ${10}^{-5}$
• D. ${10}^{-7}$

Answer 1

Answer: (C)

${10}^{-5}$

$pH=-log[H{]}^{+}=3$.

Hence, $[H{]}^{+}={10}^{-3}$.

The relationship between the dissociation constant of a weak acid and hydrogen ion concentration is, $[H{]}^{+}=\sqrt{K_a\times c}$.

Substitute values in the above equation,

${10}^{-3}=\sqrt{k_a\times 0.1}$.

$k_a=1\times {10}^{-5}$.