Colligative Properties: A Comprehensive Guide to Understanding Solution Behavior - Prof. S, Slides of Physical Chemistry

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Colligative properties

Solution of electrolytes

Colligative properties y A colligative property may be defined as one which depends on the number of particles in solution and not in any way on the size or chemical nature of the particles. y Dilute solutions containing non-volatile solute exhibit the following colligative properties:

1. Lowering of the Vapour Pressure
2. Elevation of the Boiling Point
3. Depression of the Freezing Point
4. Osmotic Pressure y Each colligative property is exactly related to any other. Thus if one property is measured, the other can be calculated. The colligative properties of dilute solutions are particularly important as these provide valuable methods for finding the molecular weights of the dissolved substances.

The vapour pressure of the pure solvent is caused by the number of molecules evaporating from its surface. ➢When a nonvolatile solute is dissolved in solution, the presence of solute molecules in the surface blocks a fraction of the surface where no evaporation can take place. This causes the lowering of the vapour pressure. Vapour pressure of the solution is determined by the number of molecules of the solvent present at any time in the surface which is proportional to the mole fraction of the solvent. That is, vapour pressure of solution ∝ mole fraction of solvent Derivation of Raoult’s Law

Derivation of Raoult’s Law n N N Ps

v n N N or Ps k

= (^) ------------ ① In case of pure solvent n = 0 and hence Now from equation (1), the vapour pressure P = k Therefore the equation (1) assumes the form- 1 0 N N n N N Mole fractionof solvent =

=

= n N n P P- P n N N 1 P P 1 n N N P P or n N N P P s s s s

=

− = −

=

= k being proportionality factor This is Raoult’s Law.

The molecular mass of a nonvolatile solute can be determined by measuring the lowering of vapor pressure (P - P s ) produced by dissolving a known weight of it in a known weight of the solvent. If w grams of solute is dissolved in W grams of the solvent, m and M are molecular masses of the solute and solvent respectively, then we have: m w Number of Molesof solute, n = M W Number of Molesof solvent, N = Determination of Molecular Mass from Vapor Pressure Lowering

Osmosis and Osmotic Pressure