Question

The pH of a $0.1$ molar solution of the acid $HQ$ is $3$. The value of the ionization constant, $K_a$ of the acid is :

• A. $3 × 10^{-1}$
• B. $1 ×10^{-3}$
• C. $1 × 10^{-5}$
• D. $1 × 10^{7}$

Answer 1

Answer: (C)

$\underset{0.1-x}{HQ} \rightleftharpoons \underset{x}{H^+} +\underset{x}{ Q^-}$
$pH = 3, [H^+] = 10^{-3}, x = 10^{-3}$
$K_{a} = \frac{\left(x\right)\times \left(x\right)}{\left(0.1 - x\right)} = \frac{\left(10^{-3}\right)^{2}}{0.1-10^{-3}} \simeq \frac{10^{-6}}{0.1} = 10^{-5}$