    Question

# For a dilute solution, Raoult’s law states that

A
The lowering of vapour pressure is equal to the mole fraction of the solute.
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B
The relative lowering of vapour pressure is equal to the mole fraction of the solute
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C
The relative lowering of vapour pressure is proportional to the amount of solute in the solution
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D
The vapour pressure of the solution is equal to the mole fraction of the solvent.
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Solution

## The correct option is B The relative lowering of vapour pressure is equal to the mole fraction of the solute.- Raoult’s law states that the partial vapour pressure of any volatile component of a solution is the product of vapour pressure of that pure component and the mole fraction of the component in the solution.- The relative lowering of vapour pressure for the given solution is the ratio of vapour pressure lowering of solvent from solution to the vapour pressure of the pure solvent.- Consider a solution of having solute, A and solvent, B with the mole fraction XA and XB respectively. Now, A is non-volatile so it won’t evaporate and thus, doesn’t contribute to the total vapour pressure of the solution. The vapour pressure of the pure solvent is P0B and that of solute A is P0A =0.- Hence, vapour pressure of the solution will be,p=P0A+(P0B−P0A)XAp=P0BXA - Now, lowering of vapour pressure is given by,ΔP=P0B−pΔP=P0B−P0BXBΔP=P0B(1−XB) - We know that sum of the mole fraction of the components in a solution is equal to unity.-Therefore, =1−XB=XA and so get,ΔP=P0BXA - The relative lowering of vapour pressure is given by,ΔPP0=P0B−pp0B=XA - Therefore, the relative lowering of vapour pressure is equal to XA - Therefore, for a dilute solution, Raoult’s law states that the relative lowering of vapour pressure is equal to the mole fraction of the solute.So, option (A) is the correct answer.  Suggest Corrections  11      Similar questions  Related Videos   Gases in Liquids and Henry's Law
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