I've been stuck on this question for awhile now. I'm having a lot of trouble figuring out how to start it.

Question

If you had added exactly 20.00 mL do d.i. water before the titration, calculate the molar concentration of OAc- would have been at the end of the titration. (Ignore the volume of the HPhthn solution)

Answer 1

What were you titrating? HOAc?
What were you titrating with? NaOH or similar?
Take the volume of base you added, add 20 mL to that and that is the final volume. The OAc^- then will be (in molarity) mols HOAc you started with divided by the final volume in L.