Question

3 g of acetic acid Ls 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 = 60 g / mol, log3 = 0.4771]
Neglect any changes in volume.
Given 6.5

Answer 1

Molar mass of acetic acid $=2 \times 12+4 \times 1+16 \times 2=60 g$
No of moles of acetic acid $=\frac{3}{60}=\frac{1}{20}=0.05$ mole
Millimoles of acetic acid in $20 ml =\frac{0.05 \times 20 \times 1000}{500}=2$ millimoles
Millimoles of $HCl$ in $20 ml =1$ millimoles
Millimoles of $NaOH =2.5$ millimoles
Millimoles of $NaOH$ remaining $=2.5-1=1.5$ millimoles
Millimoles of $NaOH =2.5$ millimoles
Millimoles of $NaOH$ remaining $=2.5-1=1.5$ millimoles
$pH = pka +\log \frac{[ Salt ]}{[ Acid ]} \\ pH =4.74+\log \frac{[3 / 2]}{[1 / 2]} \\ pH =4.74+\log (3)=4.74+0.48=5.22$