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Topic: Amphiprotic H2PO4-  (Read 19241 times)

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Offline minijumbuk

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Amphiprotic H2PO4-
« on: April 09, 2008, 01:28:52 AM »
This was a question in my half yearlies:

A solution of KH2PO4 contains the ion H2PO4-. H2PO4- is an amphiprotic substance, yet in solution, the pH is greater than 7. With relevant equations, explain why.

This is what I wrote:
H2PO4- has 2 equations, one acting as acid (donates H+), one acting as base (accepts H+)

Then I wrote equations for H2PO4- with water for acid, and showed that it produces HPO4 2-, and then another set of equations for HPO4 2-, showing that it produces both H2PO4- and PO4 3-. This means that there is a buffer effect in the second set of equations, so the acid-equation for H2PO4- does not contribute to changing pH.

Then I wrote H2PO4- with base, and showed that it produced H3PO4 and OH-. Since H3PO4- is not amphiprotic, then the final pH will be greater than 7 due to the OH- ion.

So...Am I right or wrong? =(

Offline AWK

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Re: Amphiprotic H2PO4-
« Reply #1 on: April 09, 2008, 02:34:30 AM »
Compare protolysis constants Ka2 and Kb3 to prove which process prevails
AWK

Offline Borek

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Re: Amphiprotic H2PO4-
« Reply #2 on: April 09, 2008, 03:20:39 AM »
A solution of KH2PO4 contains the ion H2PO4-. H2PO4- is an amphiprotic substance, yet in solution, the pH is greater than 7. With relevant equations, explain why.

pH of 0.1M solution is around 4.7 so well below 7. That's close to the value predicted using simplified formula 12.9:

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

You can't explain why pH is over 7 when it is below 7.

(note: mixing H3PO4 with KOH 1:1 you get KH2PO4, that's why concentrations of both are entered as 0.1 on the picture below)

Why do I have a feeling I have seen this question lately?
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