Osmotic Pressure Calculator (van t Hoff)
Calculate osmotic pressure using the van t Hoff equation.
Enter solute concentration, temperature, and van t Hoff factor to get pressure in atm, kPa, and mmHg.
Osmotic Pressure
Van t Hoff Equation pi = i × M × R × T
Where:
- pi = osmotic pressure
- i = van t Hoff factor (number of particles a solute splits into)
- M = molar concentration (mol/L)
- R = ideal gas constant = 0.08206 L·atm/(mol·K)
- T = temperature in Kelvin (K = °C + 273.15)
Van t Hoff Factor (i)
- Non-electrolytes (glucose, sucrose): i = 1 — do not dissociate
- NaCl: i = 2 (Na+ and Cl-)
- CaCl2: i = 3 (Ca2+ and 2 Cl-)
- MgSO4: i ≈ 2
- Actual i is slightly less than theoretical due to ion pairing in solution.
Osmolarity Osmolarity = i × M (in osmol/L or mOsmol/L)
Biological References
- Human blood plasma: ~7.7 atm, ~0.30 osmol/L (isotonic)
- Cell cytoplasm: ~7.7 atm — balanced with plasma
- Seawater: ~27 atm — reverse osmosis must exceed this to desalinate
Practical Example To desalinate seawater (0.6 M NaCl, i=2): pi = 2 × 0.6 × 0.08206 × 298 ≈ 29.3 atm Reverse osmosis pumps must exceed this pressure.