[TheDog]

I'm trying to work through some problems, and while I know that they aren't that difficult I just can't seem to get my head around it.  Here they are:

You prepare a solution by adding 225 mL of .0500 M H₂PO₄^- to 105 mL of .250 M HPO₄^2-.  Calculate the pH of the resulting solution.
Ka1=7.11x10^-3
Ka2=6.34x10^-8
Ka3=4.22x10^-13


A solution of o-phthalate is adjusted to pH 5.00.  Calculate the fraction of o-phthalate that exists as H₂P, HP^-, and P^2-.
Ka1=1.12x10^-3
Ka2=3.90x10^-6

Thanks in advance.

[Yggdrasil]

This wikipedia article is a good starting point:

http://en.wikipedia.org/wiki/Henderson_hasselbach