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This question already has an answer here:

Can anyone give me a push in the right direction, how do I calculate the concentration of all species present in a solution with 0.3M NaH2PO4. So the concentration of H3PO4, H2PO4 -, HPO4 2-, PO4 3- and H30+.

I already know how to approximate the ph for an amphiprotic, pH=1/2(pKa1+pKa2). Should I use the [H30+] from this calculation in any way?

I know how to set up equilibrium equations and tables, but starting with the deprotonated form of a polyprotic acid confuses me.

I cant figure out in what order i need to calculate things.

For phosphoric acid Ka1=7.6×10^-3 Ka2=6.2×10^-8 Ka3=2.1×10^-13

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marked as duplicate by Mithoron, Jon Custer, Todd Minehardt, airhuff, Gaurang Tandon Dec 3 '18 at 3:23

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Doesnt answer my question, thanks for the help though! $\endgroup$ – Steinein Nov 29 '18 at 15:23
  • $\begingroup$ Hey @Steinein How does the linked question not help you? It does seem to be very similar. If you can explain, we can reopen the question. $\endgroup$ – Gaurang Tandon Dec 3 '18 at 3:23
  • $\begingroup$ @Mithoron and etal: The linked question provide a strong idea to answer, but this question seems confusing many chemistry teachers, that they need very detailed completed answer with explaining to remediate misconception. So, the question deserve to reopen. $\endgroup$ – Adnan AL-Amleh Dec 5 '18 at 23:06
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Try look around for Bjerrum-diagrams for phosphoric acid. These diagrams are illustrations of the distribution between an acid and its conjungate base, at a different pH. Looking at those, you will soon realize that at any given pH only one acid/base system is relevant (unless we are talking about acids with very close pKa).

If you need to calculate the distribution, you must turn to the usual equation for an acid/base-equilibrium. You know [H+] from the pH and [A-]/[HA] can be reduced to one unknown since [HA] + [A-] = Ctotal

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