For the base:
B(aq) + H2O(l) → ← HB+(aq) + OH–(aq) Kb(B)
For the conjugate acid
HB+(aq) + H2O(l) → ← H3O+(aq) + B(aq) Ka(HB+)
If we add these two reactions together:
2 H2O(l) → ← H3O+(aq) + OH–(aq) Kw
From the rule of multiple equilibria, if we add reactions, then we multiply equilibrium constants:
Ka × Kb = Kw
pKa + pKb = 14.00 (only at 25 °C)
These relationships are only true for conjugate acid/base pairs.