Osmotic Pressure Calculator

Osmotic pressure is the pressure that must be applied to a solution to stop the net flow of solvent across a semipermeable membrane from a pure solvent into the solution. It is a colligative property, depending on the number of dissolved particles, and it drives processes from plant water uptake to reverse osmosis water purification.

The van 't Hoff equation is osmotic pressure = i * M * R * T, where i is the van 't Hoff factor, M is the molar concentration of solute, R is the gas constant, and T is the absolute temperature in kelvin. With M in moles per liter, R as 0.082057 liter-atmospheres per mole-kelvin, and T in kelvin, the pressure comes out in atmospheres.

The van 't Hoff factor i counts how many particles each formula unit produces in solution. Because osmotic pressure depends on the total particle concentration, a salt that dissociates into two ions (i equals 2) exerts twice the osmotic pressure of a nonelectrolyte at the same molarity. Sugar has i equal to 1.

Human blood plasma has an osmolarity of about 0.30 moles of particles per liter, which at body temperature of 310 K gives an osmotic pressure of roughly 7.6 atmospheres. Intravenous fluids are formulated to match this value (isotonic) so that red blood cells neither swell nor shrink.

Enter molar concentration in moles per liter, temperature in kelvin, and the dimensionless van 't Hoff factor. The gas constant is a user-editable input preset to 0.082057 liter-atmospheres per mole-kelvin. The osmotic pressure is reported in atmospheres and converted to kilopascals.