Question

Given the pKa of acetic acid to be 4.75. Calculate the pH of a buffer that is comprised of 50.0 mL of 0.18 M acetic acid and 75.0 mL of 0.15 M sodium acetate. THEN you add 15.0 mL of 0.03 M HCl to the buffer solution described. What are the [H+] and pH after the mixture reaches equilibrium?

Answer 1

milli moles of acetic acid = 50 x 0.18 = 9

millimoles of sodium acetate = 75 x 0.15 = 11.25

pH = pKa + log [salt /acid]

      = 4.75 + log [11.25 / 9]

      = 4.85

pH = 4.85

[H+] = 1.4 x 10^-5 M

After adding HCl :

millimoles of HCl = 15 x 0.03 = 0.45 (C).

pH = pKa + log [salt - C / acid + C]

     = 4.75 + log [11.25 - 0.45 / 9 + 0.45]

     = 4.81

pH = 4.81

[H+] = 1.56 x 10^-5 M