Question

3 g of acetic acid is added to 250 mL of 0.1 M HCl and the solution made up to 500 mL. To 20 mL of this solution $\frac{1}{2}$ mL of 5 M NaOH is added. The pH of the solution is?

[Given: pKa of acetic acid $=4.75$ , molar mass of acetic acid $=60g/mol,log3=0.4771$ ] Neglect any changes in volume.

• A. 2.61
• B. 4.52
• C. 5.22
• D. 8.78

Answer 1

Answer: (C)

mili mole of acetic acid in 20 ml $=2$

mili mole of HCl in 20 ml $=1$

mili mole of NaOH 20 ml $=2.5$

$\underset{0}{\underset{1.5}{\underset{(left)}{\mathrm{NaOH}}}}+\underset{0.5}{\underset{2}{C{H}_{3}COOH}}\to \underset{1.5}{\underset{0}{C{H}_{3}COONa}}+\underset{0}{H_{2}O}$

$pH=pKa+log\frac{salt}{acid}=4.74+log\frac{1.5}{0.5}$

$=4.74+log3=4.74+0.4771=5.22$