Equating gaseous and osmotic pressures can be very misleading. While
intuitively brilliant, van't Hoff's insight was based on an analogy between
the behavior of gas and solute and cannot be taken too literally. Why not?
Unlike gas molecules in a compartment, the particles of solute in a solution
do not themselves produce an osmotic pressure of the walls of the compartment.
Solute causes osmosis only through its effect on solvent diffusion, and
solvent diffusion, in turn, creates osmotic pressure. Moreover, the osmotic
pressure of any solution depends on two extraneous and additional factors:
the presence of another solution connected to it through a selectively permeable
membrane and the diffusion of water between these two solutions by osmosis.
Thus, a molar solution of non-electrolyte will produce 22.4 atmospheres of osmotic pressure only when it is connected by means of a selectively permeable membrane to another compartment containing distilled water! In other words, a single solution has no osmotic pressure, only an osmotic potential, and many scientists prefer thinking about osmosis in this manner.