Assertion : Lowering of vapour pressure is di | vedclass

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(B) Both assertion and reason are correct but reason is not the correct explanation of assertion.The relationship between lowering of vapour pressure

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If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.

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(B) Both assertion and reason are correct but reason is not the correct explanation of assertion.The relationship between lowering of vapour pressure and osmotic pressure can be derived as follows:Van't Hoff equation for a dilute solution is $\pi = \frac{n}{V}RT$ ..... $(i)$In the case of a dilute solution,the volume of the solution can be taken as equal to that of the solvent. If $N$ is the number of moles of solvent of molecular weight $M$ and density $\rho$,the volume $V$ is given by $V = \frac{NM}{\rho}$ ...... $(ii)$Substituting $(ii)$ in $(i)$,we get $\frac{n}{N} = \frac{\pi M}{\rho RT}$ .... $(iii)$From Raoult's law,the relative lowering of vapour pressure is $\frac{p^o - p}{p^o} = \frac{n}{N}$ ...... $(iv)$Equating $(iii)$ and $(iv)$,we get $\frac{p^o - p}{p^o} = \frac{\pi M}{\rho RT}$Rearranging,$(p^o - p) = \frac{\pi M p^o}{\rho RT}$Since $\frac{M p^o}{\rho RT}$ is constant at a constant temperature,$(p^o - p) \propto \pi$.Thus,the lowering of vapour pressure is directly proportional to the osmotic pressure. The assertion is correct.Osmotic pressure is indeed a colligative property,so the reason is also correct. However,the fact that osmotic pressure is a colligative property does not explain why the lowering of vapour pressure is proportional to it.

Assertion : Lowering of vapour pressure is directly proportional to osmotic pressure of the solution.
Reason : Osmotic pressure is a colligative property.

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Assertion : Lowering of vapour pressure is directly proportional to osmotic pressure of the solution.Reason : Osmotic pressure is a colligative property.