Question

What does the Kw = Ka + Kb formula stand for? Please give an example.

Answer 1

This equation shows a relation between an acid HA and its conjugate base A- at equilibrium, given as:
Kw = Ka x Kb

Where, Ka is the dissociation constant for an acid in water:
HA + H2O <----> H3O+ + A- . . . . .Ka = ([H3O+][A-]) / [HA]
Ka is a measure of how much H3O+ is produced. The higher the Ka, the stronger the acid.
Kb is the dissociation constant for a base in water:
B + H2O <--------> HB + OH- . . .Kb = ([HB][OH-]) / [B]
Kb is a measure of how much OH- is produced. The higher the Kb, the stronger the base.
Kw is the dissociation constant for water reacting with itself:
H2O(l) + H2O(l) <-------> H3O+ + OH- . . . .Kw = [H3O+][OH-]
Notice that H2O(l) does not appear in the denominator of Kw since it is a pure liquid. The value of Kw at 25 C = 1 x 10^-14.