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Question

Which expression of relative lowering of vapour pressure is correct:
delta P/initial vapour pressure of the solvent(P^0)
OR
DeltaP/vapour pressure of solution(Ps)


Solution

Let us see how this lowering in vapour pressure is determined mathematically:

Let us assume a binary solution in which the mole fraction of the solvent be x1 and that of the solute be x2 , p1 be the vapour pressure of the solvent and p1o be the vapour pressure of the solvent in a pure state.

According to Raoult’s Law:

p1=x1p1o…………………………..(1)

The decrease in vapour pressure of the solvent (∆p1) is given by:

=> ∆p1=p1o-p1

=> ∆p1=p1o-p1ox1                              [using equation (1)]

=> ∆p1=p1o (1-x1)                             

Since we have assumed the solution to be binary solution, x2=1-x1

=> ∆p1=p1ox2

=> x2∆p1/p1o

The above equation gives the relative lowering of vapour pressure which is equal to the mole fraction of the solute.


Chemistry

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