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 acid
• B. Hypochlorous acid > Acetic acid > Formic acid
• C. Formic acid > Hypochlorous acid > Acetic acid
• D. Formic acid > Acetic acid > Hypochlorous acid

Answer 1

Answer: (B)

Hypochlorous acid > Acetic acid > Formic acid

Answer 2

: $K_{a}$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

HCOOH > $CH_{3}COOH$ > HClO

$K_{a:}1.8 \times 10^{- 4}1.74 \times 10^{- 5}3.0 \times 10^{- 8}$

Stronger the acid, lesser will be the value of pH. Hence, the correct order of pH is

HClO > $CH_{3}COOH$ > HCOOH