How to calculate osmotic pressure

Osmotic pressure is the pressure needed to stop water from flowing into a solution across a membrane, and it depends on the number of dissolved particles, not what they are. First find osmolarity by multiplying the molarity by the van't Hoff factor i — the number of particles each formula unit splits into (glucose stays as 1, NaCl splits into Na⁺ and Cl⁻ for 2, CaCl₂ into 3). Then the van't Hoff equation gives the pressure: π = iMRT, with R = 0.08206 L·atm/mol·K and temperature in kelvin. Comparing to blood plasma (about 0.30 Osm, roughly 7.6 atm at body temperature) tells you the tonicity: a more concentrated solution is hypertonic and pulls water out of cells, a less concentrated one is hypotonic and pushes water in.

Assumes ideal, fully dissociating solutes. Real solutions deviate slightly (the osmotic coefficient), so measured values run a bit lower than the ideal i.

Related tools: Molar mass & molarity · all biochem tools.