FIGURE 11-6 Osmotic pressure. Solutions A and B are separated by a membrane that is permeable to water but impermeable to all solutes. If C_B (the total concentration of solutes in solution B) is greater than C_A, water will tend to flow across the membrane from solution A to solution B. Osmotic pressure π is the hydrostatic pressure that would have to be applied to solution B to prevent this water flow. From the van’t Hoff equation, osmotic pressure is given by π = RT(C_B − C_A), where R is the gas constant and T is the absolute temperature.