Non-ideal solutions can be studied using the vapor pressure as an observable. The vapor pressure of an ideal solution can be calculated by a knowledge of the mole fraction. If the sample is prepared by known amounts solvent and solute then one can calculate the mole fraction. Calculation of the non-ideal solution properties can be achieved using a study of the vapor pressure of the actual solution and then comapring that measurement to the theoretical value. The vapor pressure of the real solution may be either greater than or less than the ideal solution. If the solvent and solute are attracted to one another then they will have less of tendency to enter the vapor phase. on the other hand, if the solvent and solute repel each other then we would expect the vaor presure to be higher than the ideal vapor pressure. The Henry's law constant is one way to express deviation from ideality in the limit of low mole fraction. The Henry's law constant replaces the pure vapor pressure.

P_{2} = x_{2}K_{H,2}

The Henry's law constant indicates that the slope of the pressure is significantly different than what is predicted by Raoult's law. It is useful for strong deviations from ideality in solutions when the solute concentration is low. It is also used to describe solutions of gases (e.g. O_{2}, N_{2}O_{2} etc.) in soluvent.

The concentration in a non-ideal solution can be considerd as an effetive concentration, an activity. Activity is determined by measurin vapor presure and compation to the expected vapor pressure based on the ideal solution model. The activity is related to the mole fraction by an activity coefficient. The activity coefficient will have a value of 1 for an ideal solution, but may be greater or less than 1 for non-ideal solutions. Therefore, the activity coefficient is a measure of the deviation from ideality. It can be determiend based on a measurement of the actual vapor pressure relative to the theoretical (Raoult's law) vapor pressure.