Raoult's Law

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Introduction

French Chemist François-Marie Raoult found out that when substances were mixed in a solution, the vapour pressure of the solution decreased at the same time, while conducting an experiment in 1877. To explain this phenomenon Raoult proposed a law, named Raoult’s law, which also became one of the laws of thermodynamics. Let us learn more about Raoult’s law and understand its principles and applications and study about its limitations in this article.


What is Raoult’s Law?

Raoult’s law states that a solvent’s partial vapour pressure in a solution is equal or the same as the vapour pressure of the pure solvent multiplied by its mole fraction in the solution. This means that the freezing and boiling points of an ideal solution are respectively depressed and elevated relative to that of the pure solvent by an amount proportional to the mole fraction of the solute.

Mathematically, Raoult’s law equation is given by the following formula.

Psolution = Xsolvent.P0Solvent

Where,

Psolution = Vapour pressure of the solution

Xsolvent = Mole fraction of the solvent

P0Solvent = Vapour pressure of the pure solvent


Importance of Raoult’s Law

Suppose a closed container is filled with a volatile liquid A. After some time, due to evaporation, vapour particles of A will start to form. As time passes, the vapour particles of A are going to be in dynamic equilibrium with the liquid particles on the surface. So, the pressure exerted by the vapour particles of A at any particular temperature is called the vapour pressure of A at that temperature. Vapour pressure is exhibited by all solids and liquids and depends only on the sort of liquid and temperature.

Now, if another liquid B is added to this container, the B particles will occupy the space between A particles on the surface of the solution.

For any given liquid there are a fraction of molecules on the surface having sufficient energy to escape to the vapour phase.

Since we have a fewer number of A particles on the surface at this point, the number of vapour particles of A in the vapour phase will be lesser. This is going to result in lower vapour pressure of A.

Now if B is also volatile, we will have a fewer number of B particles in the vapour phase as compared to pure liquid B.

This new pressure is the partial pressure of each A and B, and is given by Raoult’s law and depends on the concentration of each component in the liquid phase.

PA ∝ XA , PB ∝ XB  = XAP’A= XBP’B

Where P’ is the mole fractions of the components


Limitations of Raoult’s Law

The limitations of Raoult’s Law are as follows.

  • Raoult’s law is apt for describing ideal solutions, that is, the solutions in which the gas phase exhibits thermodynamic properties analogous to those of a mixture of ideal gases. However, they are not easily found and are rare. Different chemical components have to be chemically identical equally.

  • Since many of the liquids in the mixture do not have the same uniformity in terms of attractive forces, these types of solutions tend to deviate away from the law postulated by Raoult or it does not follow Raoult's law appropriately.


Deviations from Raoult’s Law

Deviations from Raoult’s Law can be done if there are adhesive or cohesive forces between two liquids.

Negative Deviation

When the vapour pressure is lower than expected from the law, this results in a negative deviation. It occurs when forces between particles are stronger than those between particles in pure liquids. For instance, this behaviour is observed in a mixture of chloroform and acetone. In this case, hydrogen bonds cause deviation. A solution of hydrochloric acid and water is another example of this.

Positive Deviation

The positive deviation occurs when the cohesion between similar molecules exceeds adhesion between unlike molecules. The result is higher-than-expected vapour pressure. Both components of the mixture escape the solution more readily than if the components were pure. This behaviour is observed in mixtures of benzene and methanol, and mixtures of chloroform and ethanol.

FAQ (Frequently Asked Questions)

Q1. What is Raoult’s law?

Answer- French Chemist François-Marie Raoult found out that when substances were mixed in a solution, the vapour pressure of the solution decreased simultaneously while conducting an experiment in 1877. To explain this phenomenon Raoult proposed a law, named Raoult’s law. Raoult’s law is also recognized as one of the laws of thermodynamics.

Q2. What is the formula for Raoult’s law?

Answer- Mathematically, Raoult’s law equation is given by the formula-

Psolution = Xsolvent.P0Solvent

where,

Psolution = Vapour pressure of the solution.

Xsolvent = Mole fraction of the solvent.

P0Solvent = Vapour pressure of the pure solvent.

Q3. What are the applications of Raoult’s law?

Answer- Raoult’s law is widely used to estimate the contribution of individual components of a liquid or solid mixture to the total pressure exerted by the system.

Q4. Does Raoult’s law apply to all kinds of solutions? Explain your answer.

Answer- Raoult’s law is not applicable to all kinds of solutions but only ideal solutions. An ideal solution is the one that has solvent-solute interactions the same as the solvent-solvent or solute-solute interaction. This implies that solute or solvent, both require the same energy to escape to the vapour phase in their pure states.