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Osmotic Pressure Definition


Osmotic pressure is the pressure exerted by the movement of water molecules through a semipermeable membrane from a lower solute concentration region to a higher solute concentration region. It is a colligative property that depends on the number of solute particles in a solution rather than the type. The greater the solute particles in a solution, the higher the osmotic pressure.

What is Osmotic Pressure?

Osmotic Pressure Definition

Osmotic pressure is the pressure required to prevent the flow of water molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. This pressure is created by moving water molecules from the side of the membrane with a lower solute concentration to the side with a higher solute concentration to equalize the solute concentration on both sides of the membrane.

Osmotic pressure is a fundamental concept in many biological and industrial processes. It plays a critical role in maintaining the water balance of cells and organisms, as well as in water purification and the production of certain industrial products such as pharmaceuticals and food products.

Osmotic Pressure Equation

The osmotic pressure equation is given by:

π = iMRT


  • π is the osmotic pressure (in pascals or atmospheres)
  • i is the van't Hoff factor, which is the number of ions or particles formed by each molecule of solute (for example, a solute that dissociates into two ions in solution would have a van't Hoff factor of 2)
  • M is the molar concentration of solute (in moles per liter)
  • R is the gas constant (8.314 J/molK or 0.08206 Latm/mol*K)
  • T is the absolute temperature (in Kelvin)

This equation shows that osmotic pressure is directly proportional to the concentration of solute particles and the absolute temperature and is inversely proportional to the volume of the solution. It also demonstrates that osmotic pressure is a colligative property that depends on the number of solute particles in the solution and not on the identity of the solute particles.

Osmotic Pressure Example

Here is an example of how to calculate osmotic pressure:

Suppose we have a solution of glucose (C6H12O6) at a concentration of 0.1 M. We want to calculate the osmotic pressure at room temperature (25°C or 298 K).

First, we must find glucose's van't Hoff factor (i). Since glucose does not dissociate in solution, its van't Hoff factor is 1.

Next, we can plug the values into the osmotic pressure equation:

π = iMRT

π = (1)(0.1)(0.08206)(298)

π = 2.43 atm

Therefore, the osmotic pressure of the glucose solution at room temperature is 2.43 atm.

This means that if the glucose solution were separated from pure water by a semipermeable membrane, water would flow from the pure water side to the glucose solution side until the pressure on the glucose side reached 2.43 atm. The concentrations of solutes on both sides of the membrane were equalized.

Applications of Osmotic Pressure

Osmotic pressure has many applications in various fields, such as biology, medicine, and industry. Some of the most important applications are:

  1. Biological systems: Osmotic pressure is critical in maintaining the water balance and regulating the osmotic pressure of cells and organisms. For example, cells must maintain an osmotic balance with their environment to avoid bursting or fading due to water loss or gain.
  2. Medical applications: Osmotic pressure is used in medical treatments such as dialysis, where a semipermeable membrane removes excess solutes from the blood of patients with kidney failure. Osmotic pressure is also important in delivering certain drugs, as drug particles may be too large to pass through the membrane of cells or tissues without the help of osmotic pressure.
  3. Food industry: Osmotic pressure is used to preserve and process food. In food preservation, osmotic pressure removes water from food products to prevent bacterial growth and spoilage. In producing certain food products such as jams, jellies, and candied fruits, osmotic pressure infuses them with sugar or other solutes.
  4. Purification of water: Osmotic pressure is used in water purification in reverse osmosis systems. The semipermeable membrane used in reverse osmosis allows water to pass through while retaining solutes removed from the system.
  5. Industrial applications: Osmotic pressure is used in the production of many industrial products, such as the purification of certain chemicals and the separation of different components of mixtures. It is also used to produce certain pharmaceutical products, such as the concentration of protein solutions.

Types of Osmotic Pressure

Osmotic pressure may be broadly divided into three categories based on the solute and solvent components:

  • Isoosmotic pressure: where two solutions have the same solute concentrations and, as a result, have equal or similar osmotic pressure.
  • Hyperosmotic pressure: where a solution's higher osmotic pressure than the surrounding fluid under comparison is one of its distinguishing characteristics
  • Hypoosmotic pressure: where a solution's lower osmotic pressure than the surrounding fluid under comparison is one of its defining features

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