February 22, 2020, 03:01:56 AM
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Topic: Why can't pH of diprotic acids be calculated like that of amphiprotic salts?  (Read 1813 times)

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Offline krackers

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I was reading this website (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?

—————————————————————

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



I don't understand how they came to the conclusion that [H3AsO4-] ≈ [HAsO42-]

Offline krackers

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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?

Offline cseil

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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.

Offline Borek

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Already discussed here.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline cseil

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Sorry. I didn't see that.  :)

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