Calculating the pH of a buffer solution

Question

Calculate the pH of a buffer solution prepared by mixing 30 mL of 5% acetic acid with 10 mL of 0.5 M NaOH.

Answer 1

We can use the Henderson Hasselbalch equation:

pH = pKa + log [salt] / [acid]

Reaction:

CH3COOH + NaOH ==> CH3COONa + H2O

30 ml x 5 g CH3COOH / 100 ml = 1.5 g CH3COOH x 1 mol CH3COOH / 60.05 g = 0.0250 mols CH3COOH

10 ml x 1 L / 1000 ml x 0.5 mol NaOH / L = 0.005 mols NaOH

Final volume = 30 ml + 10 ml = 40 ml = 0.040 L

CH3COOH + NaOH ==> CH3COONa + H2O

0.025...........0.005................0................Initial

-0.005.......-0.005.............+0.005...........Change

0.02..............0...................0.005..........Equilibrium

You can use moles of convert to M by dividing moles by 0.04 L

pH = pKa + log [salt] / [acid] and pKa for acetic acid = ~4.75 (your tabular value may be different)

pH = 4.75 + log (0.005 / 0.02) = 4.75 + log 0.25

pH = 4.15