[horsebox]

In the lab in college we did an experiment where we reacted acetic acid with NaOH then took a pH reading. Then we determined the dissociation constant of the acid but theres one part thats a mystery to me. To use the henderson hesselbalch equation to get the Ka we needed to know the amount of dissociated H+ and conjugate base ions present and the way they calculated it was by writing out the balanced reaction equation then assuming that the amount of acetate ions present must equal the amount of NaOH that was used. I don't understand why the amount of dissociated acetate ions would equal the amount of NaOH that was reacted with the solution.