pKa Scale

Definition of pKa

You can think of any molecule that has a proton on it as being an acid.  They don't have to be very strong acids, but nonetheless they can behave as an acid.

If you take some arbitrary acid, H-A, and place it in a solvent usually water) it will undergo an acid base dissociation reaction as follows.  An equilibrium constant K_a can be written for this equilibrium.  A strong acid would have a large K_a.  This indicates the proton (H+) is not tiightly bound.  While a weak acid would have a very small K_a.  

Now recall from general chemistry the definition of pH = -log([H⁺]). We define pKa the same way.

       pK_a = -log(K_a)

This have been done and tabulated for many thousands of compounds.  Lets think about this equation for a minute.  Suppose you had a very strong acid.  The equilibrium above would be far to the right.  The equilibrium constant would be very large and log(K_a) would be very large.  However the -log(K_a) would be very small.

Chemists have done this for many compounds.  Below is a small subset with which you should be familair (i.e. memorize).

Table of pKa's