Question

The ionisation constant of an acid, $K_a$ is the measure of strength of an acid. The $K_a$ values of acetic acid, hypochlorous acid and formic acid are $1.74 \times 10^{-5}$, $3.0 \times 10^{-8}$ and $1.8 \times 10^{-4}$ respectively. Which of the following orders of $pH$ of $0.1\, mol \,dm^{-3}$ solutions of these acids is correct?

• A. Acetic acid $>$ Hypochlorous acid $>$ Formic add
• B. Hypochlorous acid $>$ Acetic acid $>$ Formic add
• C. Formic acid $>$ Hypochlorous add $>$ Acetic add
• D. Formic acid $>$ Acetic acid $>$ Hypochlorous add

Answer 1

Answer: (B)

$K_a$ is a measure of the strength of the acid i.e., larger the value of $K_a$, the stronger is the acid.
Thus, the correct order of acidic strength is
$\begin{matrix}&HCOOH&>&CH_{3}COOH&>&HClO\\ K_{a} :&1.8\times10^{-4}&&1.74\times10^{-5}&&3.0\times10^{-8}\end{matrix}$
Stronger the acid, lesser will be the value of $pH$. Hence, the correct order of $pH$ is $HClO > CH_3COOH > HCOOH$