🧪 Osmotic Pressure — Van't Hoff Equation & Osmometer

A membrane osmometer's capillary column rises until hydrostatic pressure balances the osmotic driving force. Van't Hoff's Π = iMRT computes the equilibrium live as you tune concentration and temperature.

ChemistryInteractive
Left: Membrane osmometer with rising capillary column · Right: Π vs M curve with current marker · Adjust concentration and solute type

How it Works

This simulation models a rigid two-chamber membrane osmometer. The left chamber holds pure solvent open to the atmosphere. The right chamber holds a solution and is connected to the left chamber through a semipermeable membrane at the bottom — permeable to solvent molecules but not to solute — and to a thin vertical capillary tube open at the top. Because the solution has a lower solvent chemical potential, solvent molecules cross the membrane net into the solution side, raising the liquid level in the capillary tube.

As the column rises, its weight creates a hydrostatic back-pressure ρgh pushing solvent back across the membrane. The column keeps climbing until this back-pressure exactly balances the osmotic driving force Π, at which point net solvent flow stops and the system reaches equilibrium. The equilibrium osmotic pressure is given by the van't Hoff equation, and the equilibrium column height follows directly from a simple pressure balance.

Van't Hoff equation: Π = iMRT
R = 0.0821 L·atm/(mol·K)
Equilibrium height: h = Π / (ρg)
Kilopascal conversion: Π(kPa) = Π(atm) × 101.325

Frequently Asked Questions

What is osmotic pressure and the van't Hoff equation?

Osmotic pressure Π is the pressure that must be applied to a solution to stop the net inflow of solvent across a semipermeable membrane. The van't Hoff equation Π = iMRT relates it to the molar concentration M, the van't Hoff factor i, the gas constant R, and the absolute temperature T — directly analogous to the ideal gas law PV = nRT.

Why is osmotic pressure a colligative property?

Colligative properties depend only on the number of dissolved particles, not on their chemical identity. Because Π = iMRT depends only on the total particle concentration iM, any two solutes with the same effective particle concentration produce identical osmotic pressure.

Why do real solutions deviate from Π = iMRT at high concentration?

At high concentration, ions and molecules interact with each other, so the solution no longer behaves ideally. This deviation is captured by an osmotic coefficient φ, giving Π = φiMRT, where φ approaches 1 in the dilute limit and drops below 1 as concentration increases.

How does reverse osmosis work?

In reverse osmosis, mechanical pressure greater than the solution's osmotic pressure Π is applied to the solution side of a semipermeable membrane, forcing pure solvent to pass back through the membrane and leaving solutes behind. It is the basis of most seawater desalination plants.

How is osmometry used to measure molar mass?

Rearranging the van't Hoff equation gives M = Π/(iRT). By measuring the equilibrium osmotic pressure of a solution containing a known mass of an unknown compound, chemists can solve for M and hence the molar mass — especially useful for polymers and proteins.

How are boiling point elevation and freezing point depression related to osmotic pressure?

Boiling point elevation, freezing point depression, and osmotic pressure are all colligative properties arising from the same cause: dissolved solute particles lower the chemical potential of the solvent. All three scale with the van't Hoff factor i and the effective particle concentration.

What are real-world uses of osmotic pressure?

Osmotic pressure principles are used in industrial water purification and seawater desalination via reverse osmosis, in food processing (osmotic drying of fruit), in pharmaceutical formulation to make IV solutions isotonic, and in forward-osmosis water and power technologies.

What determines the van't Hoff factor i for a solute?

The van't Hoff factor i is the number of independent particles one formula unit of solute produces in solution. Non-electrolytes like glucose have i = 1; NaCl dissociates into 2 ions (i = 2); CaCl₂ dissociates into 3 ions (i = 3), assuming complete dissociation.

Why does the osmometer capillary column stop rising at equilibrium?

As solvent crosses the membrane, the rising column creates a hydrostatic back-pressure ρgh. The column keeps rising until this back-pressure exactly equals Π, at which point net solvent flow across the membrane becomes zero.

About this simulation

Written by MySimulator Team · Reviewed by MySimulator Editorial Review

Last updated: 11 July 2026

This simulator models a classic membrane osmometer: a rigid apparatus where pure solvent and a solution sit in separate chambers joined by a semipermeable membrane, with a thin capillary tube rising from the solution side. As solvent crosses the membrane into the more concentrated solution, the capillary column climbs until its own hydrostatic weight pushes back hard enough to halt further net flow — the point of osmotic equilibrium. The right-hand chart tracks the same physics analytically: van't Hoff's equation Π = iMRT predicts a straight line through the origin relating osmotic pressure to concentration, with a live marker showing exactly where your chosen solute and concentration land.

🔬 What it shows

Two synchronized views of osmotic equilibrium: an animated two-chamber osmometer where the capillary column rises toward its equilibrium height as solvent particles cross the membrane, and a Π vs M line chart showing how osmotic pressure scales linearly with concentration for any chosen van't Hoff factor.

🎮 How to use

Drag the concentration slider to change M, adjust temperature to see Π = iMRT respond directly, and pick a solute type (glucose, NaCl, CaCl₂, or polymer) to change the van't Hoff factor i. The capillary cross-section slider only changes how fast the column animates toward equilibrium, since real equilibrium height depends solely on pressure balance, not tube width.

💡 Did you know?

Membrane osmometry is still used today to measure the molar mass of large polymers and proteins, because even very dilute solutions of huge molecules produce an easily measurable osmotic pressure — far more sensitive than freezing-point depression for macromolecules.

Frequently asked questions

Why does the column stop rising instead of overflowing?

As the capillary column rises, its own weight presses back down on the solution chamber through hydrostatic pressure ρgh. Once that back-pressure equals the osmotic driving force Π, the net movement of solvent across the membrane becomes zero, so the column reaches a stable equilibrium height rather than rising indefinitely.

Does the membrane let solute through at all?

No — an ideal semipermeable membrane, as modeled here, is permeable only to solvent molecules and completely blocks the solute. This selective permeability is exactly what creates the imbalance in chemical potential that drives solvent across the membrane in the first place.

Why does raising the temperature increase Π even at constant concentration?

The van't Hoff equation Π = iMRT shows osmotic pressure scales directly with absolute temperature T, just like pressure in the ideal gas law. Higher temperature means more thermal energy driving solvent molecules across the membrane, so the same concentration produces a proportionally larger osmotic pressure and taller equilibrium column.

Why does capillary tube width not affect equilibrium height?

Equilibrium height depends only on balancing pressures: Π = ρgh, which contains no term for tube cross-sectional area. A narrower tube reaches the same equilibrium height, just with less total volume of solvent needed to fill it — which is why this simulation uses the cross-section slider only to change animation speed, not the final height.

How is this different from biological cell osmosis?

This simulator is a purely physical apparatus — a rigid osmometer with fixed chambers and a real semipermeable membrane, used to quantitatively measure or predict Π. Biological cell osmosis involves flexible, living membranes with active transport and more complex selective permeability, and is typically studied qualitatively rather than by exact hydrostatic balance.

Why do NaCl and CaCl2 produce more osmotic pressure than glucose at the same molarity?

NaCl dissociates into 2 ions and CaCl₂ into 3 ions in solution, so their van't Hoff factor i (2 and 3 respectively) multiplies the effective particle concentration used in Π = iMRT. Glucose does not dissociate (i = 1), so at equal molarity, NaCl produces roughly double and CaCl₂ roughly triple the osmotic pressure of glucose.