1. The problem statement, all variables and given/known data What is the pH of the resulting solution when 100.0 mL of 0.10 M H3PO4 is mixed with 200.0 mL of 0.15 M NaOH? 2. Relevant equations Henderson-Hasselbalch, equilibrium constant expressions 3. The attempt at a solution I've thought about this question for over an hour, yet I have still gotten nowhere. I eventually came to the conclusion that the excess number of moles of OH- will dominate. Based on this conclusion, I calculated a pH of 12.82. However, this answer was marked wrong. Can anyone lend some insight into this situation?