Question

A certain mixture of HCl and CH3-COOH is 0.1 M in each of the acids. 20 ml of this solution is titrated against 0.1MNaOH. By how many units does the pH change from the start to the stage when the HCl is almost completely neutralised and acidic acid remains unreacted ? Ka for acetic acid = 2 x 10-5.

• A. 1.5
• B. 3
• C. 2
• D. 3.25

Answer 1

Answer: (C)

2

Answer 2

m. moles of HCl = 0.1 × 20 = 2

m. moles of CH3COOH = 0.1 × 20 = 2

After titration of HCl by NaOH

[CH3COOH] = $\frac { 1 } { 20 }$M

\ pH = $\frac { 1 } { 2 }$(pKa – log C) = $\frac { 1 } { 2 }$ [5 – log 2 – log ($\frac { 1 } { 20 }$)] = 3.