# Polyprotic acid - How does one calculate the CONCENTRATIONS of ALL species present in a solution when you start with 0.3M NaH2PO4? [duplicate]

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

• Doesnt answer my question, thanks for the help though! – Steinein Nov 29 '18 at 15:23
• 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. – Gaurang Tandon Dec 3 '18 at 3:23
• @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. – Adnan AL-Amleh Dec 5 '18 at 23:06