When the solute undergoes ionic dissociation in solution (for example a salt), the system becomes decidedly non-ideal and we need to take the dissociation process into consideration. One can define activities for the cations and anions separately ( and ).
In a liquid solution the activity coefficient of a given ion (e.g. Ca) isn't measurable because it is experimentally impossible to independently measure the electrochemical potential of an ion in solution.  (One cannot add cations without putting in anions at the same time). Therefore, one introduces the notions of
;mean ionic activity
;mean ionic molality
;mean ionic activity coefficient
where  represent the stoichiometric coefficients involved in the ionic dissociation process
Even though  and  cannot be determined separately,  is a measurable quantity that can also be predicted for sufficiently dilute systems using Debye–Hückel theory. For electrolyte solutions at higher concentrations, Debye–Hückel theory needs to be extended and replaced, e.g., by a Pitzer electrolyte solution model (see external links below for examples). For the activity of a strong ionic solute (complete dissociation) we can write: