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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⁺ + S^2-

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=245287d426cf3b251e1e873ff9a4c4172d9d3cdb)

I don't understand how they came to the conclusion that [H₃AsO₄^-] ≈ [HAsO₄^2-]

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 H₂AsO₄^2- considered amphiprotic when the value of K_-1, that is the equilibrium constant for H₂AsO₄^2- + 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.  :)