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 × 10^–5, 3.0 × 10^–8 and 1.8 × 10^–4 respectively. Which of the following orders of pH of 0.1 mol dm^–3 solutions of these acids is correct?

(i) acetic acid > hypochlorous acid > formic acid

(ii) hypochlorous acid > acetic acid > formic acid

(iii) formic acid > hypochlorous acid > acetic acid

(iv) formic acid > acetic acid > hypochlorous acid

Answer 1

(ii) Hypochlorous acid > Acetic acid > Formic acid

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>CH₃COOH>HClO 

Stronger the acid, lesser will be the value of pH. 

Hence, the correct order of pH is 

HClO>CH₃COOH>HCOOH.