To prepare a buffer solution with a pH of 5.10, calculate how many ml of NaOH solution should be added to 500 ml of HAc solution? (Concentration of both NaOH and HAc is 0.1 mol/L, pKa (HAc)=4.75.)

Answer 1

Henderson Hasselbalch equation

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

5.10 = 4.75 + log [Ac-]/[HAc]

log [Ac-]/[HAc] = 0.35

[Ac-]/[HAc] = 2.24

moles HAc present initially = 0.5 L x 0.1 mol/L = 0.05 moles HAc

Let x = moles Ac- and then 0.05 - x = moles HAc after reaction with NaOH (NaOH + HAc => Ac- + H2O)

x/0.05 - x = 2.24

x = 0.0346 moles Ac- = 0.0346 moles NaOH

(x L)(0.1 mol/L) = 0.0346 and x = 0.346 L = 346 mls