Ka is the Keq for the dissociation of an acid (HA + H2
O --> H3
O+ + A-). It is defined the same way as Keq with all concentrations at equilibrium. Ka can be used to identify the strength of the acid. A larger the Ka, the stronger the acid. The smaller the Ka, the weaker the acid. Strong acids are those that completely dissociate. The more dissociation of the acid, the more products (A-) are formed, the larger the numerator in the Ka equation, and therefore a larger Ka. The less dissociation of the acid, the less products are formed, the smaller the numerator in the Ka equation and therefore the smaller Ka.
Kb is the Keq for the dissociation of a base (A- + H2
O --> HA + H3
O+). Kb can be approached in the same way as the acid. The larger the Kb value, the stronger the base and the smaller the Kb value the weaker the base. Ka and Kb are related by: (Ka)(Kb) = Kw.