Intermediate Topic 310 of 1102 States of Matter & Solutions
Osmotic Pressure
The pressure needed to stop osmosis, equal to MRT.
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In-depth explanation of Osmotic Pressure
Osmotic pressure is the pressure that must be applied to a solution to stop the net flow of solvent by osmosis across a semipermeable membrane. It is a colligative property given by the van't Hoff equation Π = iMRT, where M is the molarity of the solution, R is the gas constant and T is the absolute temperature. Osmotic pressure is large even for dilute solutions, which makes it useful for measuring the molar masses of polymers and proteins. Because the value is sensitive to concentration, it can determine molar mass from very small amounts of material. Isotonic intravenous fluids are formulated to match the osmotic pressure of blood.
Going deeper: Osmotic Pressure
Key ideas: Formula Pi = i M R T, using R = 0.08206 L atm/(mol K) and T in kelvin. To find molar mass: M = (g/MW)/V so MW = g R T / (Pi V). Even 1 mg/mL of a protein of 66000 g/mol gives about 0.37 kPa, easily measurable, so the method suits large molecules. Blood is about 7.7 atm at 37 C.
Common mistakes and connections: Typical errors are using molality, forgetting i, or using Celsius. Another is mixing the units of R and the pressure. Osmotic pressure is not a force of the solution pulling water; it is the pressure that must be applied to prevent flow. Links to colligative properties, van't Hoff equation and reverse osmosis.
Visualization of Osmotic Pressure
Osmotic pressure pi = iMRT is the pressure needed to stop osmosis; a 0.1 M solution at 298 K gives 2.44 atm. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A 0.1 M solution of a nonelectrolyte at 298 K has an osmotic pressure of about 2.4 atm.
Frequently asked questions about Osmotic Pressure
What is Osmotic Pressure?
The pressure needed to stop osmosis, equal to MRT.
Is Osmotic Pressure a beginner, intermediate or advanced topic?
Osmotic Pressure is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Osmotic Pressure?
Study these earlier topics in States of Matter & Solutions first: Boiling Point Elevation, Freezing Point Depression, Osmosis.
What comes after Osmotic Pressure?
The next topics in States of Matter & Solutions are Reverse Osmosis and van't Hoff Factor.
Can I practise Osmotic Pressure with quizzes?
Yes. The interactive lesson includes a self-check question on Osmotic Pressure, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Osmotic Pressure fits in States of Matter & Solutions
Osmotic Pressure is topic 91 of 109 in States of Matter & Solutions.