Chem Problem Acetic acid/buffer

Question

I am reworking or trying to work all my questions from Acid/Base chapter. Any help would be appreciated. I am trying to get the relationship down between KB,KA KW and PH,POH,and PKA

Ka for acetic acid is 1.7 x 10-5 at 25oC. A buffer solution is made by mixing 51.4 mL of 0.127 M acetic acid with 40.1 mL of 0.172 M sodium acetate. Calculate the pH of this solution at 25oC after the addition of 5.12 mL of 0.105 M NaOH. Assume that the volumes of the solutions are additive.

Answer 1

the Ka = 1.7 X 10^-5 means the pKa = 4.77

Using th Hendersen /Haselbach relationship for buffers

pH = pKa + log base /acid

the pH is merely a ratio of the mmoles    so

mmoles acetic acid = 51.4 mL x 0.127 M = 6.528 mmoles

mmoles acetate      = 40.1 mL x 0.172M  = 6.897 mmoles

so if you are adding 5.12 mL of 0.105 M NaOH ( base ) = 0.538 mmoles then you are converting 0.538 mmoles of acetic acid into 0.538 mmoles of acetate ...so after reaction you will have

pH = 4.77 + log { 6.897 + 0.538 )/(6.528 - 0.538)} = 4.77 + log 7.435/5.99 = 4.864