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 $0.1mold{m}^{-3}$ 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
$\begin{array}{cccccc}& HCOOH& >& C{H}_{3}COOH& >& HClO\\ K_a:& 1.8\times 10^{-4}& & 1.74\times 10^{-5}& & 3.0\times 10^{-8}\end{array}$
Stronger the acid, lesser will be the value of $pH$. Hence, the correct order of $pH$ is $HClO>C{H}_{3}COOH>HCOOH$