Acid-base equilibria

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Hello, i'm confused about how to determine the pH of a polyprotic acid such as phosphoric acid. I've been trying to find how to do so on websites but I can't seem to find it!! And, I know this is more chemistry, but it is part of my biochemistry assignment and my biochem book doesn't go into detail on it!! So, what are the step-by-step equations that I would have to use to ultimately determine the pH of phosphoric acid? I have 3 pK_a's: 2.16, 7.21, and 12.32.
 
Also, when it comes to K_a equations how does the whole balancing thing work? i.e. H₃PO₄ --> 2H⁺ + HPO₄^2-
 
So, when it comes to calculating the [H⁺], do you go:
K_a = [H⁺]² [HPO₄^2-] / H₂PO₄^-]
and then to calculate the [H⁺] after this equation is solved, do you still have to multiply it by 2 since there are 2 H⁺'s for every one HPO₄^-?? Thanks!!
 

Igloo writes:
 
Fortunately there is a very easy way to carry out this calculation.
 
Given that the 1st pK_a is 2.16 and that the 2nd pK_a is 7.21, this means that the 2nd ionisation is extremely insignificant (100,000 times less significant) than that of the 1st, so you can completely ignore the 2nd pK_a. Similar reasoning shows that there is even more reason to neglect the 3rd pK_a value.
 
What I am saying effectively is that the vast majority of hydrogen ions found in aqueous phosphoric acid are derived from the first ionisation process.
 
Therefore, given that the 1st ionisation equation is H₃PO₄ ---> H⁺ + H₂PO₄^-, you can treat this acid as being monoprotic as far as pH calculations of this sort are concerned.
 
I hope that you are mightily relieved at this news!!

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updated: 14 December 2006