[alberto]

Hi, everybody! I'm an Italian student of pharmaceutical chemistry, so I apologize for my English.
However, here is my problem:
Calculate the concentration (mol/L) of HPO4-- in a buffer solution (pH: 7.30), which is 0.2 mol/L in H3P04.
I know it comes out a solution of H2PO4- , but I cannot understand how to manipulate the H&H expression in order to solve the problem. Maybe there's a formula to get the pH of such a strange buffer solution, but, INCREDIBLE AS IT MAY APPEAR, it seems to be impossible to find (neither on the web nor on my book).

[H+]=6.32E-8*([H2PO4-]/[HPO4--])

7.11E-3=[H+][H2PO4-]/[H3PO4]=x^2/(0.2-x)

6.32E-8/7.11E-3=[H3PO4][HPO4--]/[H2PO4-]^2

This is everything I worked out.

PLEASE HELP ME 'COZ THIS PROBLEM IS MAKING ME GOING CRAZY!