Colligative Properties:
The properties of the solutions which depend only on the number of solute particles present in solution but not in any way on the size or chemical nature of the solute particles are called Colligative properties.
The four important Colligative properties are —
- Relative lowering in vapour pressure
- Elevation in boiling point
- Depression in freezing point
- Osmotic pressure.
Relative lowering in vapour pressure:
The vapour pressure of a pure solvent is decrease when a non-volatile solute is dissolved in it. If PA° is the vapour pressure of pure solvent and PA that of the solution, the lowering of vapour pressure is (PA°— PA). This lowering of vapour pressure relative to the vapour pressure of pure solvent is called relative lowering of vapour pressure. Therefore,
Relative lowering of vapour pressure =(PA°- PA)/PA°
Determination of Molar Mass of a Solute from Relative Lowering in Vapour Pressure:
Molar mass of a non-volatile solute can be calculated from relative lowering in vapour pressure. According to Raoult’s law, the relative lowering in vapour pressure of a solution containing a non-volatile solute is equal to mole fraction of solute. Thus,
(PA°- PA )/ PA° =XA Where,
Or, (PA°- PA )/ PA° = nA/(nA + nB) nA = no. of moles of solute
Or, Or, (PA°- PA )/ PA° = nA/(nA + nB) nB = no. of moles of solvent
Since the law is applicable to the ideal solutions (which are very dilute), the molar concentration of the solute (WA/MA) being very small, can be neglected from the denominator. Then the above expression becomes,
Or, (PA°- PA )/ PA° = (WA/MA )/(WB/MB)
Or, (PA°- PA )/ PA° = (WAMB)/(WBMA)
Where, PA°= Vapour pressure of pure solvent, PA =Vapour pressure of pure solution, WA= Mass of solute, MA= Molecular mass of solute, WB = Mass of solvent & MB = Molecular mass of solvent.