titration curve calculation
Note: when you have added more HCl than initial amount of acetate, you may calculate pH with just excess HCl, as if the acetic acid was not present. Below pH 3.0 acetic acid is dissociated in less then 2% so its contribution to the final pH can be most likely neglected.
You may try BATE (pH calculator) to compare results of exact calculations with results of your calculation.
Borek
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pH calculator
www.pH-meter.info/pH-electrode