The more I read through the thread the less I understand.
For titration of sodium acetate with hydrochloric acid you cant use phenolopthtaleine, as the final pH will be acidic.
There are three areas of the curve. I will outline very shortly how how to deal with them, check much more detailed explanation on titration curve calculation
on my site.
pKa value of acetic acid is known and given (as AWK wrote) in physicochemical tables. Consider you know the value, it is about 4.75. The calculations can't be done without knowing Ka.
At first you have solution of sodium acetate only. Acetic anion will react with water producing OH-
ions, thus making solution slightly basic. To calculate pH you have to:
1. Calculate pKb of the acetic anion (pKa+pKb=pKw, acetic anion is a conjugated base of acetic acid)
2. Use ICE table to calculate OH-
(using Kb and ICE table)
3. Convert pOH to pH.
On the curve you have a mixture of acetic acid and acetic anion - so it is a buffer solution. Assume that all added hydrochloric acid reacted with acetic anion and from simple stoichiometry calculate concentrations of both forms, then use Henderson-Hasselbalch equation
At the endpoint all of the acetate was converted to acetic acid, so you have to calculate pH using known concentration of weak acid (don't forget the dilution factor from titration), its Ka value and ICE table.