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Osmosis is the spontaneous flow of solvent through a semipermeable membrane from dilute (low pi) to concentrated (high pi) solution. The membrane allows solvent molecules but blocks solute particles. Osmotic pressure (pi) = minimum external pressure on the concentrated side to prevent osmosis. Van't Hoff equation: pi = iMRT (M = molarity, R = 0.0821 L.atm/mol.K). Isotonic solutions have equal pi (0.9% NaCl ≈ 5.4% glucose ≈ blood). Hypertonic: higher pi — causes crenation (cell shrinking). Hypotonic: lower pi — causes haemolysis (cell bursting). IV fluids must be isotonic. Reverse osmosis: applying P > pi forces solvent from concentrated to dilute side. Applications: water desalination, ultrapure water production. Osmotic pressure advantages: (1) measured at room temperature $\frac{no heating}{cooling}$, (2) extremely sensitive for high-molar-mass solutes, (3) gives number-average molar mass. For a 1 g/L protein (M ≈ 60000): $delta_{Tf}$ ≈ 3$x10^{-5}$ K (immeasurable) but pi ≈ 0.4 cmH2O (measurable). Berkeley-Hartley method measures osmotic pressure by applying external pressure until osmosis stops — more accurate than measuring solvent rise.