Equivalence point question

[Clinicall]

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So I had a question that asked if a titration of H2A (Acid), what is the earliest point where the solution is entirely A^2-.

I understand that there will be 2 equivalence points at which [H2A <--> H+ and HA-] and [HA- <-> H+ and A^-2] in equilibrium.

Now it has 5 possible choices. 3 buffer regions (beginning, middle, and end of titration) and 2 equivalence points. It says that the answer is the second equivalence point, but wouldn't it still be in equilibrium as shown above, and not be ENTIRELY converted? I think that it should be the last option (or at least above the second EP where it would atleast be higher in concentration). What am I not doing right?

EDIT: Ok so I think my TPR book is screwing me. Tell me if this is correct or not. The EP is when a species is 100% converted, and the buffer regions (pKa region) is where there the molecule its in equilibrium with its protonated and de-protonated form? My TPR book is telling me that this is the equivalence point region.

 

[caereax]

For one thing you should check your questions for readability, but if I understand you correctly--

A di-protic acid will have two buffer regions, not three. At first buffer region H2A = (HA-).
At the first e.p. there is only (HA-).
At the second buffer region, HA- = A2-.
At the second e.p. there is only A2-.

The specific thing you are getting wrong is this: you're mistaking half equivalence for equivalence point. At half equivalence, you have a buffer at equilibrium as you've shown, H2A <-> HA-. When you're at a full equivalence point, there is no equilibrium--the analyte (stuff you're titrating) has been entirely converted from one form into another.

Hope that helps.

 

[Clinicall]

So this is in my TPR book, is what this says accurate?

http://imgur.com/WkoNDBZ

I know that this shows a triprotic acid and not a diprotic acid, but is this true that the at the first equivalence point H3A <-> H+ H2A- ?

 

[caereax]

I'm just some random guy on the internet, but:

regarding your EDIT question: you are correct. EP means 100% converted; the buffer regions are where equilibrium exists b/t protonated an un-protonated forms.

Regarding that page in TPR... not sure what is going on there. A mistake that obvious makes me think I'm not understanding something. But as far as I can tell--they're inaccurate.