Interpretation: Equilibrium expression for autoionization of water has to be written.
Concept Introduction: Autoionization of water is the reaction in which the water undergoes ionization to give a proton and a hydroxide ion. Water is a very weak electrolyte and hence it does not completely dissociate into the ions. The ionization happens to a very less extent only. The ionization of water is an equilibrium reaction and hence this has equilibrium rate constant.