Question

0.2 M each of HCl and CH₃COOH are mixed in equal volumes. 20 mL of this solution is titrated against 0.1 M NaOH. What is the change in pH when HCl is almost completely neutralised. (given K_a (CH₃COOH) = 2.0 x 10^-5).

Answer 1

Since the two solution are mixed in equal volumes therefore, the concentration of HCl and CH₃COOH in the resulting solution is 0.1 M each. 

The presence of strong acid in a mixture suppresses the ionization of weak acid therefore, before the start of the titration, pH of the solution depends only on the H⁺ ions furnished by HCl.

pH = 1 

After neutralization of H⁺ ions furnished by HCl with NaOH, volume of the solution doubles.

Concentration of acetic acid is calculated as

For a weak acid

Change in pH = 3 – 1 = 2

The pH of the solution changes by 2 units.