osmotic pressure

Meaning: Osmosis is the net movement of solvent across a semipermeable membrane toward the side with more dissolved particles. Osmotic pressure (Π) is the pressure you would have to apply to the solution side to stop that net flow. It is a colligative property, so it depends on the concentration of solute particles that cannot cross the membrane, not on what they are.

Equation: Π = iMRT, where Π is osmotic pressure (Pa, though atm is common), i is the van't Hoff factor (dimensionless, about the number of particles per formula unit), M is molarity (mol/m³, though mol/L is common), R is the gas constant (8.314 J/(mol·K), or 0.0821 L·atm/(mol·K) for M in mol/L and Π in atm), and T is absolute temperature (K).

On the MCAT: Expect a U-tube or a cell placed in a solution. You predict which way water moves (toward higher osmolarity), compare Π for 0.1 M NaCl and 0.1 M glucose (NaCl is about twice as large because i ≈ 2), or calculate Π directly. Red blood cells swell in hypotonic solutions and shrink in hypertonic ones. Applying pressure greater than Π reverses the flow, which is reverse osmosis.

Common confusion: Students think the high-Π solution pushes water out. It does the opposite: water is drawn into it. Π is the pressure needed to prevent that inflow, so the more concentrated solution has the larger Π and gains water.