In this work we have used a closed system model to derive an analytical equation
for the osmotic pressure of two component electrolyte and non-electrolyte solutions.
In order to do so, we have used an equation of state, known as the dense system
equation of state (DSEOS), for the model system. Based on the model, a new
definition is given for the osmotic pressure of a closed system. Good agreement
between the calculated and experimental osmotic pressure is obtained. Using the
results obtained for the osmotic pressure along with some exact thermodynamic
arguments, we have calculated the solvent activity for some electrolyte and nonelectrolyte
solutions. The calculated activities are found to be in good agreement with
the experiment, in such a way that for the NaCl solutions the deviations for the
calculated activities are less than 0.02% for solutions up to six molal