Osmotic Pressure Calculator
Calculate osmotic pressure using the van't Hoff equation.
Enter molar concentration, van't Hoff factor, and temperature to get osmotic pressure in atm, kPa, mmHg, or bar.
Osmotic Pressure
Osmotic Pressure and the van’t Hoff Equation
Osmosis is the movement of water molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. Osmotic pressure is the pressure that must be applied to a solution to prevent this flow — it’s a direct measure of how strongly a solution pulls water toward itself.
The van’t Hoff equation:
π = i × M × R × T
Where:
- π (pi) = osmotic pressure
- i = van’t Hoff factor (number of particles a solute dissociates into)
- M = molar concentration of the solute (mol/L)
- R = ideal gas constant = 0.08206 L·atm / (mol·K)
- T = absolute temperature in Kelvin (K = °C + 273.15)
The van’t Hoff factor (i) for common solutes:
| Solute | i value | Reason |
|---|---|---|
| Glucose, sucrose | 1 | Does not dissociate |
| NaCl (table salt) | ~2 | Dissociates into Na⁺ and Cl⁻ |
| CaCl₂ | ~3 | Dissociates into Ca²⁺ and 2 Cl⁻ |
| MgSO₄ | ~2 | Dissociates into Mg²⁺ and SO₄²⁻ |
| AlCl₃ | ~4 | Dissociates into Al³⁺ and 3 Cl⁻ |
Worked example: A 0.1 mol/L NaCl solution at 25°C (298.15 K), i = 2: π = 2 × 0.1 × 0.08206 × 298.15 = 4.89 atm (495 kPa)
Real-world applications:
- IV fluids — saline solutions are designed to match blood osmotic pressure (~7.7 atm) to avoid cell damage
- Reverse osmosis — water purification systems apply pressure exceeding osmotic pressure to force water through membranes
- Cell biology — cells in hypertonic solutions shrink (crenation); in hypotonic solutions they swell and may lyse
- Food preservation — high sugar/salt concentrations create osmotic pressure that inhibits bacterial growth