Assertion : Lowering of vapour pressure is di | vedclass

Name: Exam Paper Generator | JEE NEET Paper Generator | Vedclass
Brand: Vedclass
Rating: 5 (35 reviews)

Question

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

Vedclass · Accepted Answer

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

Vedclass · Answer

(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.

List-$I$	List-$II$
$(A)$ Azeotrope	$(I)$ $\Delta T_b = i K_b m$
$(B)$ Henry's law	$(II)$ $p = K_H x$
$(C)$ Cryoscopic constant	$(III)$ $\Delta T_f / m$
$(D)$ Van't Hoff factor	$(IV)$ Deviation from Raoult's law
	$(V)$ $\pi = CRT$

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

Similar Questions

An aqueous solution contains $6\%$ and $4\%$ by weight of a weak acid $(HA)$ and urea respectively. If the depression in freezing point is $\frac{13.6}{3} \, ^{\circ}C$,calculate the $K_a$ of $HA$. [Given: $K_f = 1.8 \, K \, kg/mol$,Molecular weight of $HA = 60$,$d_{solution} = 1 \, g/mL$]

Match the following.

List-$I$ List-$II$

$(A)$ Azeotrope $(I)$ $\Delta T_b = i K_b m$

$(B)$ Henry's law $(II)$ $p = K_H x$

$(C)$ Cryoscopic constant $(III)$ $\Delta T_f / m$

$(D)$ Van't Hoff factor $(IV)$ Deviation from Raoult's law

$(V)$ $\pi = CRT$

The correct answer is

$P_A = (235y - 125xy) \, \text{mm of Hg}$. $P_A$ is the partial pressure of $A$,$x$ is the mole fraction of $B$ in the liquid phase in the mixture of two liquids $A$ and $B$,and $y$ is the mole fraction of $A$ in the vapour phase. Then $P^o_B$ in $\text{mm of Hg}$ is:

If the mole fraction of the solvent in a solution decreases,then:

Vedclass Test Series

Exam Paper Generator

Online Exam Module

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