Question

A buffer solution that is 0.10 M sodium acetate and 0.20 M acetic acid is prepared. Calculate the initial pH of this solution. The Ka for CH3COOH is 1.8 x 10-5 M. As usual, report pH to 2 decimal places.

Calculate the pH when 21.1 mL of 0.014 M HCl is added to 100.0 mL of the above buffer.

Answer 1

1)

concentration of acetic acid = 0.20 M

concentration of sodium acetate = 0.1 M

pKa = -log Ka = -log (1.8 x 10^-5)

pKa = 4.74

pH = pKa + log [salt / acid]

    = 4.74 + log [0.1 / 0.2]

pH = 4.44

2)

millimoles of HCl = 21.1 x 0.014 = 0.2954

millimoles of acid = 100 x 0.2 = 20

millimoles of salt = 100 x 0.1 = 10

pH = pKa + log [10 - 0.2954 / 20 + 0.2954]

     = 4.74 + log [10 - 0.2954 / 20 + 0.2954]

     = 4.42

pH = 4.42