Question

How much 5.50M NaOH must be added to 630.0mL of a buffer that is 0.0210M acetic acid and 0.0260M sodium acetate to raise the pH to 5.75?

Answer 1

pH = pKa + log(acetate/acid)

pKa = 4.75

initially:

acetate = MV = 0.026*630 = 16.38

acid = MV = 0.021*630 = 13.23

after adding X ml of base:

acetate = 16.38 + V*5.5

acid = 13.23 - V*5.5

then

pH = pKa + log(acetate/acid)

5.75 = 4.75 + log((16.38 + V*5.5)/(13.23 - V*5.5))

solve for V

10^(5.75-4.75) = (16.38 + V*5.5)/(13.23 - V*5.5))

10* ((13.23 - V*5.5)) = (16.38 + V*5.5)

132.3 -55V = 16.38 + 5.5V

-60.5V = 16.38 -132.3

V= (16.38 -132.3)/(-60.5) = 1.916 ml

add 1.916ml of M = 5.5 acid