What Happens to pH When H3PO4 is Mixed with NaOH?

[CaptainZappo]

Homework Statement

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?

Homework Equations

Henderson-Hasselbalch, equilibrium constant expressions

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?

 

[chemisttree]

Calculate the number of equivalents of H+ that can be produced by 100 mL of 0.1 M H3PO4. Calculate the number of moles of OH- in 200 mL of 0.15 M NaOH.

The reaction is:

H+ + OH- -------> H2O

It looks like the same question as "What is the pH of a solution of 0.01 moles Na3PO4 in 300 mL water?"

 

[chemisttree]

Let me rephrase my original post. How many potential moles of protons can be generated by 0.100 L of of 0.1 moles/liter of H3PO4? Compare this to the number of available moles of OH-.

After that, consider answering the question, "What is the pH of a solution of 0.01 moles Na3PO4 in 300 mL water?"