polyprotic ph problem

Zealduke

A .05 M sample of K2HPO4 was dissolved in 800 mL of water, and the pH was adjusted to 7 by using a 1 M solution of H3PO4 followed by water bringing the volume up to 1 L. How many mL of H3PO4 were added? (On this problem charge and mass balance have to be used.)

My problem is just in the very beginning, as my book is terrible, Im not entirely sure on how to approach this problem.

Im assuming the mass balance for H3PO4 will be:

[H3PO4] + [H2PO4- ] + [HPO42-] + [PO43-] = 1 M

And the charge balance for K2HPO4 would be:

[H+] + [K+] = [OH-] + [H2PO4-] + 2*[HPO42-] + 3*[PO43-]


I'm really uncertain whether I'm missing anything or what the next step should be.

Borek

Please try these lectures on pH calcualtion as a starting point.

At pH 7.0 you can ignore presence of H₃PO₄ and PO₄^3-. I have a gut feeling that this question can be solved just using Henderson-Hasselbalch equation and stoichiometry.

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