Vapor Pressure Lowering, Osmotic Pressure, and the Elementary Pseudo-Gas Model

Abstract

Guided by the apparently mysterious identity of the constant R in the equation describing the pressure of ideal gases (Boyle's law: P = NRT/V) and the osmotic pressures of dilute solutions (van't Hoff's law: π = NRT/V), it is argued that the “pseudo-gas” model is the appropriate method for the elementary discussion of dilute solutions. By introducing the concept of “distending pressure” (due to the thermal agitation of the solute molecules), it is shown that this model accounts for the vapor pressure lowering of solutions. Indeed, Rauold's law, now often taken as the basis of discussion, can be derived in the limit of infinite dilution and ideality. The approach also exemplifies how apparently disparate phenomena can be unified on an elementary level.