Phosphoric acid, H3PO4(aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. Estimate the pH, and the concentrations of all species in a 0.250 M phosphoric acid solution.

[H3PO4] = ?
[H2PO4^-] =?

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  1. Tell me what you don't understand about this.

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  2. i converted from pKa to Ka for all three values. i don't know where to go from there.

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  3. What about looking at the three Ka values seprately? You notice that they are about 10^5 from each other. That means that the (H^+) is determined primarily by the first ionization. The second and third ionization produce little H^+. So do the first calculation as you would a monoprotic acid and (at least for the time being) ignore the second and third. Do that and see if you can see where to go from there.

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  4. Ka1 is relatively large; probably you will need to solve the quadratic for Ka1.

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