Weak bases: Ammonia

Weak bases, like weak acids, react with water to establish an equilibrium system. Ammonia is a typical weak base. It reacts with water to form the ammonium ion and the hydroxide ion:

Like a weak acid, a weak base is only partially ionized. There’s a modified equilibrium constant expression for weak bases — the Kb. You use it exactly the same way you use the Ka (see  Acetic acid and other weak acids” for the details), except you solve for the [OH^–].