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A question regarding ionization constants of strong acids

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Organishe:
Hello all,

Something I have wondered for a while is why is there such variation in the reported values of the pKas for strong acids? For example, in one of my organic books, it is reported that the pKa  of HI in water is -10. In another book, it is reported as -7, and in yet another it is -11. I have found the same thing for HCl and HBr in various books.  For a weak acid such as acetic, though, the value listed is nearly always the same to a couple of decimals.  So why is there such variation? If I had to guess, I would say because it is a strong acid, the amount that remains in the unionized form is so small that experimental error has a significant effect on the result.

In many cases its not like it would matter, since it is so strong to begin with. Just curious....

Thanks all,

Eric

Borek:

--- Quote from: Organishe on June 11, 2006, 09:08:42 PM ---If I had to guess, I would say because it is a strong acid, the amount that remains in the unionized form is so small that experimental error has a significant effect on the result.
--- End quote ---

This, plus fact that some of these constants are not determined experimentally, but calculated using approach similar to Born-Haber cycle.

dls:
Take into consideration that the reading of the pH electrode, when less than about 1 and when more than about 13, is more semantic than real.  See also a book by the late Prof. J Ricci "Hydrogen Ion Concentration."

dls

Mitch:
Also check out our "pKa values" link to your left in the menu

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