Question

Calculate the pH of 1.00 L of the buffer 0.95 M CH₃COONa/0.98 M CH₃COOH before and after the addition of the following species. (Assume there is no change in volume.)
(a) pH of starting buffer:
(b) pH after addition of 0.070 mol NaOH:
(c) pH after further addition of 0.126 mol HCl:

Answer 1

a)

pH = pKa + log(A-/HA)

pH = 4.75 + log(0.95/0.98)

pH =

b)

mol of acid left = 0.98 -0.07 = 0.91

mol of acetated formed = 0.95 + 0.07 = 1.02

pH = pKa + log(A-/HA)

pH = 4.75 + log(1.02/0.91 )

pH = 4.7995

c)

mol of acid left = 0.98 -0.07 = 0.91

mol of acetated formed = 0.95 + 0.07 = 1.02

then, after adding another 0.126 mol of acid:

mol of acid left = 0.91 +0.126 = 1.036

mol of acetated formed = 1.02-0.126 =0.894

pH = pKa + log(A-/HA)

pH = 4.75 + log(0.894/1.036)

pH = 4.685