A buffer solution is made by mixing 40 cm3 of 0.200 mol dm-3 ethanoic acid (pKa 4.76) with 100 cm3 of 0.100 mol dm-3 sodium ethanoate. Calculate the pH of this mixture.

Answer 1

Use the Henderson Hasselbalch equation: pH = pKa + [salt]/[acid]

Final volume = 40 ml + 100 ml = 140 ml = 0.140 L

Final [ethanoic acid] = 0.040 L x 0.200 mol/L = 0.008 moles/0.140 L = 0.0571 M

Final [Na ethanoate] = 0.1 L x 0.100 mol/L = 0.01 moles/0.140 L = 0.0714 M

Substituting into the HH equation we have...

pH = 4.76 + log (0.0714/0.0571)

pH = 4.76 + 0.097

pH = 4.86