Unit – V B

Laws of Osmotic Pressure:

van’t Hoff’s Theory of Osmotic Pressure:

  • He found that the solute particles in dilute solutions possess kinetic energy and move in a random directions in the solutions. Thus they have similar behaviour as that of gas molecules.
  • On collision against semipermeable membrane the solute molecules exert osmotic pressure equal to the pressure which the solute molecules would exert if it were gas molecule at the same temperature and occupying the same volume as that of solution.
  • Thus the gas laws are equally applicable to dilute solutions.

van’t Hoff’s Boyle’s Law of Solution:

  • At constant temperature the osmotic pressure (π) of a dilute solution is directly proportional to its molar concentration (C) or inversely proportional to volume (V) of the solution.
  • Explanation:

Osmotic Pressure 01

van’t Hoff’s Charle’s Law of Solution:

  • The concentration remaining constant, the osmotic pressure (π) of a dilute solution is directly proportional to absolute temperature (T) of the solution.
  • Explanation:

Osmotic Pressure 02

van’t Hoff’s General Solution Equation:

  • By van’t Hoff Boyle’s law at constant temperature the osmotic pressure (π) of a dilute solution is inversely proportional to volume (V) of the solution.

  •  By van’t Hoff Charles law, The concentration remaining constant, the osmotic pressure (π) of a dilute solution is directly proportional to absolute temperature (T) of the solution.

Osmotic Pressure 04



  • Where k is proportionality constant called general solution constant. van’t Hoff further proved that this constan k is equal to universal gas constant R

Equations (a) and (b) represents general solution equation.

van’t Hoff’s Avogadro’s Law of Solution:

  • Two solutions of equal concentrations of different solutes exert same osmotic pressure at the same temperature OR equal volumes of isotonic solutions contain an equal number of solute particles at the given temperature.
  • Explanation:

Osmotic Pressure 06

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