The hydronium ion notation is convenient in explaining why acids
are divided into two categories, strong (complete dissociation in
water) and weak (incomplete dissociation).
Two equilibria compete for the proton when a general acid, HA,
dissociates into H+ and A-:
One can regard A- and H2O as two B-L bases
in competition for the proton, with the H2O having the
unfair advantage of being present in great excess.
If the acid anion, A-, is a stronger B-L base than
the water molecule, it will have a greater attraction for protons.
It will be able to compete successfully with water moleules for
the available protons. Most of the acid then will exist as HA instead
of A-, and we will call HA a weak acid.
In contrast, if H2O is a stronger B-L base than is A-,
the attraction of water molecules for protons will be greater than
that of A- ions. H2O will take protons away
from HA and form hydronium ions (hydrated H+).