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Show that relative lowering of vapour pressure is a colligative property.

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According to Raoult’s law,

Psolution xA , where xA = mole fraction of solvent. 

Psolution = k . xA , where k = proportionality constant 

For a pure solvent,

Vapour pressure = P°, xA = 1 

solution = k x 1 = k 

Substituting P°solvent in Raoult’s law 

Psolution = P°solvent . xA 

Relative lowenng of vapour pressure

substituting Psolution as P°xB in the above eaquation Relative lowering of vapour pressure

xA + xB = I 

where xB = mole fraction of solute. It is clear that the relative lowering of vapour pressure depends only on the mole fraction ofthe solute (xB) and is independent of its nature. Therefore relative lowering of vapour pressure is a colligative property.

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