Many organic species are very weakly acidic in water. Hence, their acid dissociation constants are small and tedious to work with. The quantity pKa is an attempt to solve this problem.
pKa of a species is defined as follows:
pKa = –logKa
By definition, the larger the Ka of a species, the smaller its pKa.
As is evident from the above numbers, it is more convenient to work with pKa values than with Ka values when Ka values are very small.