Colligative Property 1

The colligative properties are also called the democrative properties. These are discussed as follows:-

  1. Relative lowering of vapor pressure:-

According to Raoult’s Law the vapour pressure of a solution containing non-volatile solute is given by the formula

pS = xB X P0B ---------------(1)

where xB = mole fraction of solvent and p0B = vapour pressure of solvent in pure state. If xA is the mole fraction of solute.

                xA + xB = 1                            or xB = 1 – xA

Substituting in equation (1)

                pS = P0B (1 – xA)

Where WA and MA are the weight and molar mass of solute and WB and MB are the weight and molar mass of solvent.

Since the law is applicable to dilute solutions for which WA << WB, hence WA can be


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