Amphiprotic KH2PO4 (1 Viewer)

[minijumbuk]

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? =(

 

[dan.121212]

Question 21 of the 2007 CSSA trial:

"with the aid of appropriate equations, explain why the dihydrogen phosphate ion (H2PO4-) is amphiprotic yet an aquesous solution of KH2PO4 has a pH greater than 7" (5 marks)

What the suggested answers say is basically to get those 5 marks, u need :
-an equation showing H2PO4- acting as a base,
-an equation showing it acting as an acid,
-defining/explaining what 'amphiprotic' means (ie ability to donate AND accept a proton)
-explain that when H2PO4- reacts with water, as it is a STRONGER BASE than water, it will be the proton acceptor, forming OH- ions
-"relate a pH> 7 to basicity"

So yeh, i rekon u answered it well, although i dunno about the buffer part

 

[dan.121212]

in a nutshell - you're right!

 

[minijumbuk]

Hmm...Ok, cool =P
xD Thanks

 

[Undermyskin]

Dan's answer is awesome.

I think the buffer part is so facetious...basically wrong, I suppose. You can't include anything 'buffer' in this sort of question when the presence of the conjugate base or acid is not mentioned. I think you misinterpreted the question and the buffer.

 

[minijumbuk]

Well, I actually didn't talk about the buffer part, but I was alluding to it. I just said that "the acid part of the reaction with water with H2PO4- does not contribute to the pH change"
Haha

 

[Undermyskin]

Ha...ha...?

What do you mean by 'not contribute to pH change'?

 

[minijumbuk]

cuz there's no specific creation of H+ or OH- if HPO4- ionises in water, so i said there would be no pH change... =\

very high chance i'm wrong