Osmotic Pressure
Pressure required to stop solvent flow across a semipermeable membrane
The Formula
Π = i M R T
Use T in Kelvin; R = 0.08206 L·atm/mol·K (or 8.314 J/mol·K with Pa)
Variables
- Π = osmotic pressure
- i = van't Hoff factor
- M = molarity (mol/L)
- R = gas constant
- T = temperature (K)
When to Use
- Determining molar mass from osmotic pressure
- Biological membranes and cells
- Reverse osmosis and water purification
Step-by-Step Example
Problem:
Calculate the osmotic pressure of 0.20 M glucose solution at 25 C. (Assume i = 1)
1) Convert temperature
T = 25 + 273.15 = 298.15 K
2) Apply Π = i M R T
Π = 1 × 0.20 mol/L × 0.08206 L·atm/mol·K × 298.15 K
Π = 4.89 atm
Answer:
Osmotic pressure is 4.89 atm.
Common Mistakes to Avoid
Using Celsius
Always convert temperature to Kelvin.
Wrong R value
Match R units to pressure units (atm vs Pa).
Ignoring i
Electrolytes have i greater than 1; nonelectrolytes use i = 1.
High concentration
At higher M, deviations from ideal behavior occur; use activity coefficients.
Related Calculators
Frequently Asked Questions
What is i for NaCl?
Ideal i = 2; effective i about 1.8-1.9 at moderate concentration.
Can osmotic pressure find molar mass?
Yes, solve for M from Π = iMRT then back-calculate moles and molar mass.
Does membrane type matter?
Must be semipermeable to solvent only; otherwise measured Π is lower.