Question

A beaker with 175 mL of an acetic acid buffer with a pH of 5.000 is sitting on a benchtop. The total molarity of acid

and conjugate base in this buffer is 0.100 M. A student adds 6.80 mL of a 0.490 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased.

Answer 1

mmol of HCl added = MV = 6.8*0.49 = 3.332 mmol of HCl

then we need mmol of acetic acid and acetate

from pH equation

pH = pKa + log(acetate/conjugate)

since pKa = 4.75 and pH = 5

then

5 = 4.75 + log(acetate/acetic acid)

ratio = 10^*(5-4.75) = 1.778

acetate = 1.778*acid

if acetate = 0.1 then

acid = 1.778

then

mmol of aceteic acid = 1.778*175 = 311.15

mmol of acetate = 0.1*175 = 17.5

then, after adding 3.332 mmol of HCl

mmol of aceteic acid = 311.15+3.332 = 314.482

mmol of acetate = 17.5-3.332 = 14.168

then

pH = 4.75 + log(14.168/314.482)

pH = 3.403

the pH decrase = 5-3.4 = 1.6 pH units decreae i.e -1.6