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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: krackers on July 24, 2014, 01:07:58 AM

Title: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?
Post by: krackers on July 24, 2014, 01:07:58 AM
I was reading this website (http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt (http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt)) on calculating pH of amphiprotic salts.

My question is why can't you do something similar for diprotic acids? Take for example the diprotic acid H2S.

Because you have the following reactions:

 H+ + HS-  ::equil:: H2S

HS- ::equil:: H+ + S2-

can't you  treat HS- like an amphiprotic substance and calculate the pH the same way you calculated pH for other amphiprotic substances?

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I also have another question relating to these topics. In this image,

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi.stack.imgur.com%2FFrfxa.png&hash=6c5ba328411e6feaf4ae4fc525ae30e4)

I don't understand how they came to the conclusion that [H3AsO4-] ≈ [HAsO42-]
Title: Re: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?
Post by: krackers on July 24, 2014, 11:24:42 PM
I don't see a way to edit posts so I would also like to append another question to the above post. Why is H2AsO42- considered amphiprotic when the value of K-1, that is the equilibrium constant for H2AsO42- + H+  ::equil:: H3AsO4, is much higher than that of the reaction where it acts an acid?
Title: Re: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?
Post by: cseil on August 11, 2014, 04:10:09 PM
You can't.
The first reaction (H2S + H2O <=> HS- + H3O+) isn't quantitative.
You will have a remaining part of H2S in solution, you must consider that part.

Talking about the second question, well.. you have to do the proton balance.
You have H2AsO4 into water.

H3AsO4 <= H2AsO4- => HAsO4--  => AsO4---

You can disregard AsO4--- (because of it's very very small concentration).
The last question has the same answer of the first. It is a weak acid, even if the constant is high, the reaction is not quantitative.
Title: Re: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?
Post by: Borek on August 11, 2014, 05:22:50 PM
Already discussed here (http://www.physicsforums.com/showthread.php?t=763470).
Title: Re: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?
Post by: cseil on August 12, 2014, 03:04:59 AM
Sorry. I didn't see that.  :)