What Is the pH After Titrating Butanoic Acid with NaOH?

[nobb]

Hi.
I am having trouble with acid-base titration excercises. 0.02 L butanoic acid @ 0.1 M is titrated with 0.01 L NaOH solution @ 0.1 M. The Ka of butanoic acid is 1.54X10^-5. Find the pH. The answer is 4.81. I know how to find the pH when no NaOH is added, but I really don't see how the Ka fits in when adding a base. An explanation would really be appreciated. Thanks.

 

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=titration-curves-calculation

First you have a weak acid solution, then it is a buffer so you should use Henderson-Hasselbalch equation and finally you have conjugate base solution with pH defined by hydrolyzis.