of water to be expressed as:
:

In dilute aqueous solutions, the activities of solutes (dissolved species such as ions) are approximately equal to their concentrations. Thus, the ionization constant, dissociation constant, self-ionization constant, water ion-product constant or ionic product of water, symbolized by K_w, may be given by:
:where [H₃O⁺] is the molarity (molar concentration) of hydrogen cation