buffer question, pls help?

Question

A buffer solution is prepared by adding 0.250mol of KOH(s) to 1.0L of 0.200M H3PO4 solution. pH of the buffer solution is....?
Thanks a lot. I'm doing a review for my exam tomorrow when i stumbled at this question

Answer 1

The first 0.2 moles of KOH will react with 0.2 moles of H3PO4, turning it into (H2PO4)-. The remaining 0.05 moles of KOH will react with that, forming 0.05 moles of (HPO4)2-, leaving 0.15 moles of (H2PO4)-. Now you have the number of moles of acid and base, and a total volume. You need to look up the Ka of (H2PO4)-, and you're on your way!