Equating gaseous and osmotic pressures can be very misleading. How so?
While the particles of solute in a solution produce a pressure of the walls of the
compartment as they collide with it, they generate an osmotic pressure indirectly,
mainly through their effect on the activity of water molecules.. The differential
diffusion of water, in turn, creates osmotic pressure. Thus, the osmotic pressure of
any solution depends on two extraneous factors (in addition to those characterizing
gaseous pressure) : the presence of another solution connected to it through a selectively
permeable membrane and the diffusion of water between these two solutions by osmosis.
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 exerts no effective osmotic pressure, only an osmotic potential, and many scientists prefer thinking about osmosis in this manner.