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Raoult’s Law - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

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15 Questions around this concept.

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A solution of two miscible liquids showing negative deviation from Raoult’s law will have ?

decreased vapour pressure, increased boiling point

increased vapour pressure, decreased boiling point

decreased vapour pressure, decreased boiling point

increased vapour pressure, increased boiling point

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Identify the mixture that shows positive deviations from Raoult's Law

$\mathrm{CHCl}_3+\left(\mathrm{CH}_3\right)_2 \mathrm{CO}$

$\mathrm{CHCl}_3+\mathrm{C}_6 \mathrm{H}_6$

$\left(\mathrm{CH}_3\right)_2 \mathrm{CO}+\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2$

$\left(\mathrm{CH}_3\right)_2 \mathrm{CO}+\mathrm{CS}_2$

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Concepts Covered - 2

Non-Ideal Solution Showing Negative Deviation from Raoult's Law

These solutions have vapour pressure less than that predicted by Raoult’s law for the entire range of composition.

This happens when the new solute-solvent interactions are stronger than the interactions in the pure components. Since the newly formed forces are stronger than the existing forces, heat is liberated. Hence, the enthalpy change in the mixing of these solutions is negative i.e $\Delta H_{mix} < 0.$

The change in the volume during the mixing process is positive i.e. $\Delta V_{mix} < 0.$ . For example, if 1 litre solutions each of liquid A and B are mixed, then the solution obtained has a volume lesser than 2 litres.

The entropy of mixing is positive as new interactions are introduced into the solution which increases the randomness of the system and hence, The mixing process is spontaneous and hence $\Delta G_{mix} > 0.$

These solutions have vapour pressure greater than that predicted by Raoult’s law.

$P_A < P{_{A}^{o}}X_A$
$P_B < P{_{B}^{o}}X_B$
$P_T = P_A + P_B <P{_{A}^{o}}X_A+ P{_{B}^{o}}X_B$

Examples of solutions showing negative deviation:

Acetone + Chloroform
Nitric acid HNO3 + water
Acetic acid + pyridine
Phenol + Aniline

Non-Ideal Solution Showing Positive Deviation from Raoult's Law

Solutions showing positive deviation from Raoult’s law

These solutions have vapor pressure greater than that predicted by Raoult’s law for the entire range of composition.

This happens when the new interactions are weaker than the interactions in the pure component (A-B < A-A or B-B interactions). Since the newly formed forces are weaker than the existing forces, heat has to be supplied to break old bonds and form new ones. Hence, the enthalpy change in the mixing of these solutions is positive i.e $\Delta H_{mix} > 0.$

The change in the volume during the mixing process is positive i.e. $\Delta V_{mix} > 0.$ For example if 1 litre solutions each of liquid A and B are mixed, then the solution obtained has a volume greater than 2 litres.

The entropy of mixing is positive as new interactions are introduced into the solution which increases the randomness of the system and hence $\Delta S_{mix} > 0.$ . The mixing process is spontaneous and hence $\Delta G_{mix} < 0.$

These solutions have vapor pressure greater than that predicted by Raoult’s law.
$P_A> P{_{A}}^{o}X_A$
$P_B> P{_{B}}^{o}X_B$
$P_T=P_A+P_B> P{_{A}}^{o}X_A+P{_{B}}^{o}X_B$