K_A (acid constant) and K_B (base constant)

acid solutions
HA	+	H2O		H3O+	+	A-
acid		base		acid		base
A- is conjugated base of the acid HA
KA = acid constant
If KA is very large (or pKA is very small), HA is a very strong acid
An equilibriium always will shift to the side of the weaker ones.

basic solutions
A-	+	H2O		HA	+	OH-
base		acid		acid		base
HA is conjugated acid of the base A-
KB = base constant
If KB is very large (or pKB is very small), A- is a very strong base
An equilibriium always will shift to the side of the weaker ones.