Osmosis & Osmotic Pressure Calculator

Osmosis Guide

What's the key thing to understand about 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).

What do I need to know about 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 potential, i.e. into the surrounding solution) — water always moves down its potential gradient, from the less negative (higher) value toward the more negative (lower) value, regardless of which side has the higher solute concentration in absolute terms.

What do I need to know about 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 plasmolysis (the cell membrane pulls away from the cell wall entirely as the cell loses turgor completely) — this is why wilted plants in dry soil can often recover with watering (cells regain turgor) but severe, prolonged plasmolysis causes permanent cell damage.

What do I need to know about 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 not the larger waste molecules being removed. As blood flows past the membrane, waste solutes like urea diffuse down their concentration gradient into the dialysis fluid, while the fluid's composition is carefully controlled so essential solutes like glucose and electrolytes are retained in the blood rather than lost across the membrane.