d McQuare Peter A.Rock Ethan Gallogly Now consider an amphotenc salt that comes

Question

d McQuare Peter A.Rock Ethan Gallogly Now consider an amphotenc salt that comes from a triprotic acild like phosphoric. We have to look carefully at the two relevant pKa values. pka1 pka2 pka3 2.16 7.21 12.32 What is the pH of a 0.12M NazHPO4 solution? The pH for such a solution is given by But which pKa values? pH If the solution where more concentrated, like 0.22M, would the pH change? Number yes Ono Previous Give Up & View Solution OCheck Answer 0Next- Hint about us careers |partners privacy policy terms of use |contact us 18 MacBook Air

Explanation / Answer

The compound Na2HPO4- is a salt, and itf you just saw the anion: HPO42- we realize that this compound, as it comes from the phosphoric acid, means that the other possible compound that it can form, from this anion would be PO42- so:

HPO42- + H2O <-------> PO43- + H3O+   pKa3 = 12.32

Obviously the previous reaction from this would be:

H2PO4- + H2O <-------> HPO42- + H3O+   pKa2 = 7.21

So this are the two pka values to use:

pH = 12.32 + 7.21 / 2 = 9.765

Hope this helps

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