MadSci Network: Chemistry

Re: pKa of water is not 7 ???

Date: Wed Aug 4 18:40:37 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Chemistry
ID: 1091556258.Ch

Here is the first step to convincing you that the pKa of water should not be 7:

water is clearly a weaker acid than hydrogen sulfide, whose pKa is 7.04, or boric acid, 9.14, or 
hydrogen cyanide, 9.31

The real problem is why the pKa should not be 14.0 -- the same as pKw

Here is the reason: Kw is defined as [H+]*[OH-], and this turns out to be 1.E-14.

For any acid HA in water, though, Ka is defined as [H+]*[A-]/[HA]

So if we think of water as an acid, Ka = [H+] * [OH-] / [H2O] = 1.0E-14 / [H2O]

What do we do for [H2O]? Well, 1 g of water occupies 1 mL, near enough, which means that 18 
gram of water = 1 mol of water occupies 18 mL. So we can fit 55.6 mol of pure water in 1.00 L.

It is therefore fair to say that the "concentration" of water is 55.6 M, and so Ka = 1.8E-16

The log of 1.8E-16 to base 10 is -15.75, which is the pKa value quoted in your question.

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