Relationship between Ka and Kb
The reciprocal link between the strengths of an acid and its conjugate base.
Formula / equation
Ka × Kb = Kw
Unit
Dimensionless
In depth
Multiplying the two dissociation equilibria gives the self-ionisation of water, so their constants multiply to Kw. It follows that pKa + pKb = 14 at 25 °C, and that the stronger an acid is, the weaker its conjugate base must be.
Examples in the real world
Ethanoic acid's Ka of 1.8 × 10⁻⁵ gives its acetate conjugate a Kb of 5.6 × 10⁻¹⁰.