Re: Diprotic Acid- Partial Neutralization Calculation
farooq_w_at_hotmail.com
Date: 11/12/04
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Date: 11 Nov 2004 20:18:25 -0800
Bob wrote:
>
> I have at least browsed the posts so far. I basically agree with
> Wilco, including that the stated problem is ambiguous.
Thanks to both. Did you ever encounter this particular type of
questions becuase I checked Daniel Harris, Skoog and West, Underwood
and Day, Laitinen (which is fairly advanced), Vogel almost all general
chem. books but could not find a similiar solved problem. The
equilbrium expresssions in Laitinen-Chemical Analysis were very
complex.
Possibility-I: The problem is itself amibiguous and you can judge that
how painful are poorly worded questions. After reading your post we can
now assume this meaning:
"At what pH is the titration of oxalic acid 90 percent complete?". This
would simply imply the first equivalance point has been crossed and we
are some-way between the first and the second equivalence point. So we
can use now Henderson-Hasselbalch equation
pH = pKa2 + log [HA-]/[H2A]
and using the assumption that 1.8 mol of NaOH has been added determine
HA(-) and remianing H2A. Perhaps this was meant by the questioner.
> Your suggestion that [A(2-)]/[H2A] = 0.9/0.1 is "odd", in that it
> ignores the middle species HA-.
Possibilty-II;
Consider a buffer with 0.9 mol sodium oxalate and 0.1 oxalic acid.
Would then my assumption of [A2-]/[H2A] correct? Still the middle
species HA(-) is not there and I can not simply put this ratio in HH
equation.
The book from which gave the ammonia problem suggests two chapters from
Kolthoff's Treatise on Anal.Chem on Acid-Base equilibrium (by R.G
Bates) for further reading. The library clerk simply refuses (for the
sake of his comfort) that this treatise is not in the library even
though it is listed in the catalogue (supposed to be the biggest
library of Pakistan) and nobody can verify that since the reader is not
allowed to read the books directly from the shelves instead a request
slip is given to that clerk.
>
> Perhaps the intent is that H2A = 0.1, or perhaps that 1.8 OH- have
> been added per original H2A. Who knows. Perhaps the instructor who
> wrote the question has provided his/her students some groundrules
> (conventions) for parsing h/h English.
This very instructor thinks that we are poor in English!
>
>
> Back to the possibility of interpreting this as [A(2-)]/[H2A] =
> 0.9/0.1. Because the two pKs are far apart, you cannot have large
> amounts of both H2A and A2- present at the same time. So the
condition
> you suggest could only occur at an intermediate condition, when there
> is mostly HA-. I think.
>
> bob
Thanks again.
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- In reply to: Bob: "Re: Diprotic Acid- Partial Neutralization Calculation"
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