Osmosis & Osmotic Pressure Calculator
Calculate osmotic pressure using the van't Hoff equation, water potential in plant cells, and predict the direction of osmosis between two solutions.
Osmosis Guide
Osmotic Pressure — van't Hoff Equation
π = iMRT. Where π = osmotic pressure (Pa), i = van't Hoff factor (number of particles per formula unit), M = molar concentration (mol/m³ — note: 1 mol/L = 1000 mol/m³), R = 8.314 J/mol·K, T = temperature in Kelvin. For 0.1 mol/L NaCl (i=2) at 25°C: π = 2 × 100 × 8.314 × 298 = 495,874 Pa = 4.96 atm ≈ 0.50 MPa. Blood plasma osmotic pressure ≈ 0.78 MPa (approximately 300 mOsm/L). Sea water ≈ 2.7 MPa — which is why drinking sea water dehydrates rather than hydrates (draws water out of cells).
Water Potential
Water potential (ψ) determines the direction of water movement: ψ = ψs + ψp. ψs = solute potential (always negative — solutes reduce water potential). ψs ≈ −iMRT (same as −π). ψp = pressure potential (positive in turgid plant cells, zero or negative in wilted cells). Pure water: ψ = 0 MPa (reference). Water moves from HIGH water potential to LOW water potential (down the gradient). A cell with ψ = −0.6 MPa surrounded by solution with ψ = −0.3 MPa: water moves OUT of the cell (lower water potenti
Osmosis in Biology
Red blood cells in different solutions: isotonic solution (ψ = cell ψ): no net water movement — normal biconcave disc shape. Hypotonic solution (ψ_solution > ψ_cell): water moves IN — cell swells and may lyse (burst). Hypertonic solution (ψ_solution < ψ_cell): water moves OUT — cell shrinks (crenation). Plant cells: hypotonic environment — water moves in, pressure potential builds up against cell wall, cell becomes turgid. In hypertonic solution: water leaves, cell becomes flaccid, then plasmoly
Osmosis Applications
Reverse osmosis (RO): applying pressure greater than osmotic pressure forces water through a semipermeable membrane from concentrated to dilute solution — opposite to natural osmosis. Used for: desalination of sea water (pressure of 6-8 MPa needed), water purification, kidney dialysis. Dialysis: artificial kidneys use a semipermeable membrane to remove waste solutes from blood. Blood (high waste solute) is separated from dialysis fluid (low waste) by a membrane permeable to small molecules but n
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