π = iMRT
Osmotic pressure (π) is the minimum pressure needed to prevent solvent from flowing through a semipermeable membrane from a dilute to a concentrated solution. The van't Hoff equation π = iMRT relates it to the van't Hoff factor (i), molarity (M), gas constant (R), and temperature (T). Blood has an osmotic pressure of ~7.7 atm at 37°C (equivalent to 0.9% NaCl — normal saline). Reverse osmosis (applying P > π) forces solvent through the membrane against the concentration gradient — this is how seawater desalination works (seawater π ≈ 27 atm, so RO pressure = 55-80 atm). Osmotic pressure is the most sensitive colligative property — even dilute polymer solutions show measurable π, making it useful for determining molar masses of macromolecules.