Osmotic Pressure Calculator

Osmotic pressure is the pressure required to stop solvent from flowing across a semipermeable membrane into a more concentrated solution.

Osmotic pressure depends on the number of dissolved particles. A salt that dissociates into multiple ions raises the osmotic pressure in proportion to the van't Hoff factor.

For a dilute solution, osmotic pressure is the van't Hoff equation: pi = i M R T, where i is the van't Hoff factor (particles per dissolved unit), M is molarity in mol/L, R is the gas constant 0.08206 L atm/(mol K), and T is the absolute temperature in kelvin. Using R in liter-atmosphere units gives pi directly in atmospheres, which the calculator also converts to kilopascals.

Molarity (mol/L) counts moles of solute per liter, while osmolarity (osmol/L) counts moles of dissolved particles per liter. They are linked by the van't Hoff factor: osmolarity = i x molarity. A 1 mol/L glucose solution (i = 1) is 1 osmol/L, but a 1 mol/L NaCl solution (i = 2) is about 2 osmol/L because it dissociates into two ions. Osmotic pressure depends on osmolarity, not on molarity, which is why this calculator asks for the van't Hoff factor.

These terms compare a solution's osmolarity to that of the cells placed in it. An isotonic solution has the same osmolarity, so there is no net water movement (0.9% saline is isotonic with blood). A hypotonic solution is more dilute, so water flows into the cell and can make it swell or burst. A hypertonic solution is more concentrated, so water leaves the cell and it shrinks. Water always moves toward the side with the higher osmolarity.