Ka and Kb Relationship MCQ - Practice Questions with Answers

The relation between K_a,K_b and K_w can be understand by the following reaction.

Base                      Conjugate acid

The equilibrium constant K_b for base NH₃ is given as:

Again, the reaction is:

The equilibrium constant K_a for conjugate acid NH₄⁺ is given as:

Thus, K_a x K_b = [H₃O⁺][OH^-] = K_w
Thus, K_a ∝ 1/K_b at a given temperature

Therefore, if K_a of acid increases, then K_b of the conjugate base decreases. In other words, the conjugate base of strong acid is a weak base and vice-versa.