  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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