Osmotic Pressure Calculator

Calculate the osmotic pressure of a solution across a semipermeable membrane based on its molarity, temperature, and the Van't Hoff factor.

Van't Hoff Factor (i)

Solution Molarity (M)

mol/L

Temperature (T)

Osmotic Pressure (Π)

3.670 atm

Pressure in kPa371.9 kPa

Pressure in Torr / mmHg2789.1 Torr

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The Force of Water

Osmosis is the biological and chemical process where pure solvent (usually water) naturally flows across a semipermeable membrane to dilute a highly concentrated solution on the other side.

The cell walls in your body, the roots of a plant, and the filters in water purification plants all rely on osmosis.

What is Osmotic Pressure?

As water rushes across the membrane to dilute the salty side, it causes the fluid level on the salty side to physically rise, fighting against gravity. The exact amount of mechanical pressure required to push down on the salty side and completely stop the flow of water is called the Osmotic Pressure ( $\Pi$ ).

The Equation

Osmotic pressure behaves almost exactly like a gas, which is why its formula is nearly identical to the Ideal Gas Law ( $PV = nRT$ ).

\begin{aligned} \Pi = i \cdot M \cdot R \cdot T \end{aligned}

Where:

\Pi

Osmotic Pressure (atm)

Van't Hoff Factor

Molarity of Solution (mol/L)

Ideal Gas Constant (0.08206 L·atm/mol·K)

Temperature (Kelvin)

The Danger of Osmosis in Biology

If you inject pure, distilled water directly into a patient's veins, the high concentration of salts inside their red blood cells will cause massive osmotic pressure. Water will violently rush into the cells to dilute them, causing the cells to swell and explode (cytolysis). This is why IV bags always contain a 0.9% Saline solution—to match the osmotic pressure of the blood perfectly.

Frequently Asked Questions

If you apply an immense mechanical pressure to the salty side of the membrane—a pressure greater than the natural Osmotic Pressure—you can force the water to flow backward, leaving the salt behind. This is how desalination plants turn ocean water into drinking water.

Unlike boiling and freezing point equations, osmotic pressure occurs at a specific, constant temperature (like body temperature). Because the temperature isn't changing, the volume of the liquid doesn't fluctuate, making Molarity ( $mol/L$ ) safe to use.

If the soil contains too much salt, the osmotic pressure reverses. Instead of water flowing into the roots, the water is sucked out of the plant to dilute the salty soil, causing the plant's cells to collapse.

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