Osmotic pressure also depends on the solute concentration. When a solu- tion is separated from pure water by a semipermeable membrane that permits the passage of water molecules but not solutes, water tends to move into the solution in order to equalize its concentration on both sides of the membrane. Osmosis is the movement of solvent across the membrane from a region of high concentration (here, pure water) to a region of relatively low concentra- tion (water containing dissolved solute). The osmotic pressure of a solution is the pressure that must be applied to the solution to prevent the inward flow of water; it is proportional to the concentration of the solute (Fig. 2-13). For a 1 M solution, the osmotic pressure is 22.4 atm.