Question

Use the Henderson Hasselbalch equation to calculate the pH of a buffer solution prepared by dissolving 4.200 g of sodium acetate and adding 8.5 mL of 6.0 M acetic acid in enough water to prepare 100.0 mL of solution.        

Answer 1

equation for ionization of acetic acid and for Ka is

CH3COOH(aq) + H2O H3O+ + CH3COO- ......................(1)

rearanging the equation to solve for hydronium ion concentration

[H3O+ = Ka [ CH3COOH]/[CH3COO-].....................(2)

second determining the No.of moles of acid and conjugate base

6.00M CH3COOH*8.5 ml *(1L/1000 ml) = 0.051 moles of CH3COOH

4.2 ms of CH3COONa * (1mol/82.03 gms) = 0.051 moles of CH3COONa

substituting these values into the equation 2 along with pKa value of acetic acid which is 1.7*10-5

[H3O+] = 1.7*10-5 *0.051/0.051

[H3O+] = 1.7*10-5

pH = -log [ H3O+]

pH = -log 1.7 + -log10-5

pH = 0.230 + 5.00

pH = 5.23