Importance of Raoult’s law

According to Raoult’s law, the partial vapour pressure of a component in an ideal mixture is equal to the vapour pressure of the pure component multiplied by its mole fraction.

An ideal mixture is one in which all molecules have equal intermolecular interactions.

Mathematical expression

According to Raoult’s law

Where,

• Psolution  is the vapour pressure of the solution

• Xsolvent is the mole fraction of the solvent

• Po is the vapour pressure of the pure solvent.

For a solution consisting of two components A and B, the equation can be given as:

Also, we know that;

Where,

• is the mole fraction of component A

• is the mole fraction of component B

• is the vapour pressure of component A

• is the vapour pressure of component B

• is the total vapour pressure

Importance of Raoult’s law

Raoult’s Law states that at any given temperature, the saturated vapour pressure of a solution will be lower than that of the pure solvent. This has a significant impact on the solvent’s phase diagram. 

Difference between Raoult’s law and Henry’s law

The dissolution of a gas in a liquid solvent, such as water, can be explained using Henry’s law. The behaviour of a solvent in a solution that is in equilibrium with its vapour pressure is described by Raoult’s law. 

When applying these laws to real-world problems, however, there are some restrictions. 

The main distinction between Henry’s Law and Raoult’s Law is that Henry’s Law describes the behaviour of solutes in a solution, while Raoult’s Law discusses the behaviour of the solvent in a solution.

Limitations of the law

• Raoult’s law is especially useful since it explains ideal solutions, which are ones in which the gas phase has thermodynamic properties that are similar to a combination of ideal gases. The only problem is that they are rare and difficult to come by.

• Many solutions deviate from Raoult’s law due to the lack of chemical equivalency between distinct chemical components. As a result, do not follow it appropriately.

Deviation from Raoult’s law

• Positive deviation with an example

In non-ideal solutions, the partial pressure of component ‘A’ in the mixture of ‘A’ and ‘B’ is greater than that calculated from Raoult’s law. The partial vapour pressure of component ‘B’ can be higher than calculated, similar to Raoult’s law.

Water and ethanol, chloroform and water, ethanol and CCl4 , benzene and methanol, acetic acid and toluene, acetone and ethanol, methanol and H2O are examples of positive divergence from Raoult’s law.

for positive deviation.

• Negative deviation with an example

The partial vapour pressure of component ‘A’ is found to be lower than calculated using Raoult’s equation when the second component ‘B’ is added. 

When A and B are combined, the partial vapour pressure of the solution drops below that of an ideal solution of the same composition.

Such a solution has a somewhat higher boiling point than A and B, respectively.

This type of deviation from optimum behaviour is referred to as a negative departure from Raoult’s rule.

Examples include chloroform and acetone, chloroform and methyl acetate,  H2O and HCl , H2O  and HNO3, acetic acid and pyridine, and chloroform and benzene.

for negative deviation

Application of Raoult’s law

It is useful in the determination of vapour pressure involving non-volatile solute. To determine the bonding strength of liquids.

Conclusion

The molecules on the surface cause the drop in vapour pressure as the solute concentration rises. All the molecules on the surface of a pure solvent are solvent molecules. These are the surface molecules that will dissolve into the vapour phase. There are fewer solvent molecules on the surface to escape to the vapour phase as the solute concentration rises. Solute molecules have taken up some of those surface patches. The entire vapour pressure is reduced as a result.