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Osmotic Pressure: Overview, Questions, Preparation

Atoms and Molecules 2023 ( Atoms and Molecules )

Salviya Antony

Salviya AntonySenior Executive - Content

Updated on Oct 12, 2023 15:43 IST

Osmotic pressure is the minimum pressure needed to prevent the inward flow of a solution’s pure solvent through a semipermeable membrane. Osmotic pressure is a colligative property of solutions, which is a measure of the pressure required to prevent the flow of a solvent into a solution through a semipermeable membrane. It arises from the tendency of solvent molecules to move from an area of lower solute concentration to an area of higher solute concentration through a semipermeable membrane to equalise the concentration on both sides of the membrane.

Osmotic pressure is an important topic in NCERT Class 12 Chemistry. Students will learn this topic in Class 12 Chemistry chapter Solutions. In this article students will learn Osmotic pressure definition, Van't Hoff's equation and key points. 

Table of Contents
  1. Osmotic Pressure: Van't Hoff equation
  2. Osmotic Pressure: Key points
  3. Applications of Osmotic Pressure
  4. FAQs on Osmotic Pressure

Osmotic Pressure: Van't Hoff equation

Dutch chemist Jacobus Van’t Hoff put forwarded the relationship between the osmotic pressure of a solution and the molar concentration of its solute.

The mathematical relationship between osmotic pressure (π), solute concentration (c), and a gas constant (R) is described by the Van't Hoff equation:
π = icRT
Where:

  • π is the osmotic pressure
  • c is the molar concentration of the solute in solution (in moles per liter)
  • R is the ideal gas constant
  • T is the absolute temperature (in kelvin)
  • i is the Van't Hoff factor.

Note that Van't Hoff equation only holds true for solutions which behave like ideal solutions.

Osmotic Pressure: Key points

Semipermeable Membrane: Osmosis occurs through a semipermeable membrane that allows the passage of solvent molecules but not solute molecules. Common examples of semipermeable membranes include cell membranes and certain synthetic membranes.

Concentration Gradient: Osmotic pressure is directly proportional to the difference in solute concentration between the two sides of the membrane. The greater the difference in solute concentration, the higher the osmotic pressure.

Units: Osmotic pressure is typically measured in units of pressure, such as pascals (Pa) or atmospheres (atm).

Importance: Osmotic pressure is a crucial concept in biology and chemistry. In biological systems, it plays a role in processes like the movement of water in and out of cells. In chemistry, it is used to determine the molar mass of unknown solute particles in a solution through experiments.

Colligative Property: Osmotic pressure is considered a colligative property because it depends on the number of solute particles rather than their chemical nature. For example, a solution of sugar (a non-ionic solute) and a solution of salt (an ionic solute) will exert the same osmotic pressure if they have the same concentration of solute particles.

Applications of Osmotic Pressure

  • Osmotic pressure is used in water purification. This process is applicable in wastewater remediation and extracted salt from sea water.
  • Osmometry is used to determine the molecular mass of polymers. 
  • Osmotic pressure is commonly seen in plants. The leaves and stems of plants become dry and sag soon, if they run out of water. In such cases, if we provide sufficient water, plants quickly absorb it and bulge. The process that causes this is osmosis, making water flow to the salts in plant cells. The cells, in turn, inflate and grow healthy. This is a continuous process observed during plant growth. As the cells absorb more water, they ensure healthy plant growth. Thus osmotic pressure plays a key role in plant growth.  
  • Osmotic pressure plays an important role in maintaining cell homeostasis. 
  • The measurement of osmotic pressure is used to determine molecular weights of compounds.

FAQs on Osmotic Pressure

Q: At 300 K, 36 g of glucose present in a litre of its solution has an osmotic pressure of 4.98 bar. If the osmotic pressure of the solution is 1.52 bars at the same temperature, what would be its concentration?

A: Here,

T = 300 K

π = 1.52 bar

R = 0.083 bar L

Applying the relation, π = CRT

where

π = osmotic pressure of solution

C = concentration of solution

R = universal gas constant

T = temperature

⇒C  = π / RT = 1.52 / 0.083 X 300

⇒ C = 0.061mol/L

Concentration of the solution is 0.061mol/L

Q: Determine the amount of CaCl2 (i = 2.47) dissolved in 2.5 litre of water such that its osmotic pressure is 0.75 atm at 27° C.

A:

Given-

Vant Hoff’s factor, i = 2.47

osmotic pressure, π = 0.75 atm

Volume of solution = 2.5L.

To determine the amount of CaCl2, we use vant Hoff’s equation for dilute solutions, given as,

πV = inRT

where, n is the number of moles of solute, R is solution constant which is equal to the gas constant and T is the absolute temperature.

Hence, the amount of CaCl2 dissolved is 3.425g

Q: Determine the osmotic pressure of a solution prepared by dissolving 25 mg of K2SO4 in 2 litre of water at 25° C, assuming that it is completely dissociated.

A: Given-

Mass of K2SO4, w = 25 mg = 25 X 10-3 g,

Molar mass of K2SO4 = (39×2) + (32×1) + (16×4) = 174 g mol-1

Volume V = 2 liter

T = 250C + 273 = 298 K (add 273 to convert in Kelvin)

The reaction of dissociation of K2SO4 is written as,

K2SO4 → 2K + + SO42-

Number if ions produced = 2 + 1 = 3, hence vant Hoff’s factor, i = 3

Here, we use vant Hoff’s equation for dilute solutions, given as,

πV = inRT

where, n is the number of moles of solute, R is solution constant which is equal to the gas constant(0.082) and T is the absolute temperature (298 K).

Hence, the osmotic pressure of a solution is 5.27x10-3atm

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