Osmotic Pressure Calculator
Calculate the osmotic pressure of a solution from molarity, temperature, and the van ’t Hoff factor.
Osmotic Pressure (π)
Calculation Steps
What Is Osmotic Pressure?
Osmotic pressure is the minimum pressure that must be applied to a solution to stop the inward flow of solvent across a semipermeable membrane. It is a colligative property, meaning it depends on the number of dissolved particles rather than their chemical identity. The more particles a solute releases in solution, the greater the osmotic pressure.
The van 't Hoff Equation
For a dilute solution, osmotic pressure is calculated with the van 't Hoff equation:
π = i × M × R × T
- • π = osmotic pressure (atm)
- • i = van 't Hoff factor (number of particles per formula unit)
- • M = molar concentration (mol/L)
- • R = gas constant = 0.08206 L·atm/(mol·K)
- • T = absolute temperature (K)
The van 't Hoff Factor (i)
The van 't Hoff factor accounts for how many particles a solute splits into when dissolved. Non-electrolytes stay as single molecules, while ionic compounds dissociate into multiple ions.
| Solute | Ideal i | Particles |
|---|---|---|
| Glucose (C₆H₁₂O₆) | 1 | Non-electrolyte |
| NaCl | 2 | Na⁺ + Cl⁻ |
| CaCl₂ | 3 | Ca²⁺ + 2 Cl⁻ |
| AlCl₃ | 4 | Al³⁺ + 3 Cl⁻ |
Note: The van 't Hoff equation assumes an ideal, dilute solution. Real electrolyte solutions often show an effective i slightly below the ideal value because of ion pairing. Make sure the temperature is entered in Kelvin (K = °C + 273.15) so the result is consistent with R = 0.08206 L·atm/(mol·K).