Osmotic Pressure Formula, Equation, Applications, Examples

Osmotic Pressure is the pressure needed to prevent a solution's pure solvent from flowing inward through a semipermeable membrane. In addition to being known as the osmosis index, it measures how inclined a solution is to absorb pure solvents. A potential osmotic pressure is the maximum osmotic pressure a solution can create if it is separated from its pure solvent by a semipermeable membrane.

During osmosis, two solutions with varying solute concentrations are separated by a selectively permeable membrane. As solvent molecules move from a low-concentration solution to a high-concentration solution, they will continue to move through the membrane until equilibrium is reached.

Osmotic Pressure Equation

An osmotic pressure must be applied to a solution at a minimum pressure in order to stop the flow of solvent molecules through a semipermeable membrane (osmosis). The osmotic pressure is determined by the concentration of solute particles in the solution. The formula for calculating osmotic pressure is as follows:

π = iCRT

Where,

π represents the osmotic pressure

i represents van't Hoff factor

C represents the molar concentration of the solute

R represents the universal gas constant

T represents  temperature

The relationship between osmotic pressure and molar concentration of a solution was proposed by Jacobus van't Hoff. It is important to note that this equation only applies to ideal solutions.

What Exactly is Osmosis?

The phenomenon where solvent molecules pass through a semipermeable membrane from low concentration to high concentration is known as osmosis. This process eventually leads to equilibrium between the two sides of the membrane. The membrane itself selectively allows solvent molecules to pass through while blocking solute particles. If we apply sufficient pressure on the solution side, this osmosis process can be stopped. This minimum amount of pressure needed is called osmotic pressure and it ensures that the concentrations of the two solutions are equal. If enough pressure is applied, no additional water will be able to pass through the membrane, resulting in the same result.

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Understanding the Osmotic Pressure

Consider a U-Tube featuring an osmotic pressure diagram, also known as the Osmotic Pressure diagram. On the left side of the U-tube is an aqueous solution, while pure water is on the right side. The aim is for the pure water to dilute the solution by passing through a semipermeable membrane. As more water accumulates on the left side, it exerts enough pressure to stop osmosis. This is what we refer to as Osmotic pressure - the force needed to prevent inward movement of water across a semipermeable membrane. One approach to stopping osmosis is by increasing hydrostatic pressure on the solution side of the membrane, which compacts solvent molecules and boosts their "escaping tendency." Eventually, this escaping tendency equals that of molecules in pure solvent, resulting in equilibrium and ceasing of osmosis. In summary, achieving osmotic equilibrium requires applying osmotic pressure.

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Examples and Applications

With the help of osmotic pressure, plants maintain their upright shape. When the plant receives sufficient water, its cells (which contain several salts) absorb water and expand, increasing the pressure on the cell walls. Plants stand upright when they expand their cell walls.

Insufficient water supply causes hypertonic cells to wilt and lose their upright posture due to osmotic pressure.

The process of reverse osmosis is another important application of osmotic pressure in desalination and purification of seawater.