For a single salt solute with molal activity (), the osmotic coefficient can be written as where  is the stochiometric number of salt and  the activity of the solvent.  can be calculated from the salt activity coefficient via:
Moreover, the activity coefficient of the salt  can be calculated from:
According to Debye–Hückel theory, which is accurate only at low concentrations,  is asymptotic to , where I is ionic strength and A is the Debye–Hückel constant (equal to about 1.17 for water at 25 °C). 
This means that, at least at low concentrations, the vapor pressure of the solvent will be greater than that predicted by Raoults law. For instance, for solutions of magnesium chloride, the vapor pressure is slightly greater than that predicted by Raoults law up to a concentration of 0.7 mol/kg, after which the vapor pressure is lower than Raoult's law predicts. For aqueous solutions, the osmotic coefficients can be calculated theoretically by Pitzer equations or TCPC model.