Unit – III A – 01

Lowering of Vapour Pressure

Colligative Properties: 

  • Colligative properties are those properties of solutions that depends only on the number of solute particles ( atoms or molecules, ions or aggregates of molecules) in the solution and not on the nature of solute particles.
  • These properties are important because they are used to determine molar masses of non-electrolyte solutes.
  • There are four colligative properties of solution
    a) lowering of vapour pressure.
    b) elevation of the boiling point of the solvent in the solution.
    c) depression in the freezing point of the solvent in the solution.
    d) osmotic pressure

Concept of Vapour Pressure:

  • If a container is partially filled with a liquid, a portion of liquid evaporates to fill the remaining volume of the container with vapour.
  • The molecules of liquid evaporated are in continuous random motion, they collide with the walls of the container and with each other. Thus they create pressure on the walls of the container and on the liquid. At the same time some molecules which have left liquid return back to the liquid, the process is called condensation. After some interval of time, an equilibrium is established between the two phases of substance. At this stage, the rate of evaporation is equal to the rate of condensation.
  • The pressure exerted by the vapours of the liquid on the surface of the liquid when equilibrium is established between liquid and its vapour is called vapour pressure of the liquid.
  • The temperature at which vapour pressure of the liquid is equal to the external pressure is called boiling temperature at that pressure.

Lowering of Vapour Pressure:

Lowering of Vapour Pressure

  • Liquids at a given temperature vapourize and under equilibrium conditions, the pressure exerted by the vapours of the liquid over the liquid phase is called vapour pressure [Fig (a)].
  • In a pure liquid, the entire surface is occupied by the molecules of the liquid. If a non-volatile solute is added to a solvent to give a solution [Fig. (b)], the vapour pressure of the solution is solely from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature.
  • In the solution, the surface has both solute and solvent molecules; thereby the fraction of the surface covered by the solvent molecules gets reduced. Consequently, the number of solvent molecules escaping from the surface is correspondingly reduced thus, the vapour pressure is also reduced.
  • The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature.

Relative Lowering of Vapour Pressure:

The relative lowering of vapour pressure for the given solution is the ratio of vapour pressure lowering of solvent from solution to the vapour pressure of the pure solvent.
Mathematically, the relative lowering of vapour pressure is given by

p10= Vapour pressure of pure solvent
Δp= Lowering of vapour pressure
p= Vapour pressure of solution


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