Re: Does dilution change the acid ionization constant of a weak acid?

The ionization constant does not change.  Yes, the equilibrium is shifting
as the solution becomes diluted; the ionization constant tells us just
how much the equilibrium shifts.  Because of the different strengths
of acids, some will ionize more than others.  LeChatelier's principle
is at best a qualitative description of what is going on in solution.  It
doesn't attempt to address how much is happening.

Take a look at the equation:

K(a)=[H][A]/[HA] 

Each species (the unionized acid, the anion A, and the proton H) is at
a certain concentration at equilibrium.  For laughs, let's say that 
K(a) is 10e-5, making pK(a)=5.

Now we toss in a bunch of water.  Immediately all the concentrations 
are messed up, but as more ionization occurs and equilibrium is 
re-established, the final concentrations will be such that K(a) is
still equal to 10e-5.  

(remember, there is water present as well, which is also ionizing
into H+ and OH-, so the concentration [H] does not change in the same
proportion as [A])