Figure %: The autoionization of water

Knowing that Kw = 1 x 10-14 is useful because the relationship between Ka and Kb for a conjugate acid-base pair is Ka * Kb = Kw. Therefore, you can calculate the Ka of the conjugate acid of a base when given its Kb. This point is proved in below.

Figure %: The autoionization constant of water equals the product of the acid and base dissociation constants of a conjugate acid-base pair.

From the proven expression, we see that strong acids (large Ka's) form weak conjugate bases and weak acids form strong conjugate bases. In the following section, we will discuss acid and base strength in more depth.