Question

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

• A. 5
• B. $5 × 10^{-8}$
• C. $1 × 10^{-5}$
• D. $1 × 10^{-10}$

Answer 1

Answer: (D)

$K_{a} = \frac{[H^{+})^{2}}{C-[H^{+})}, \left[H^{+}\right] < < C$
$= \frac{\left(10^{-5}\right)^{2}}{1} = 10^{-10}$