**Raoult's Law**

**Raoult's Law: **The French Chemist, Francois Marte Raoult in the year 1886 while conducting an experiment found out the that the vapour pressure of the solution decreases when substances are mixed in a solution. To explain this he proposed a law called Raoult's Law. Raoult's Law states that **"For volatile solution, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution". **

**Raoult's Law Equation**

As we know, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in the solution. So Mathematically, Raoult’s law equation can be written as;

**P(solution) ∝ X(solvent)**

**P(solution) = Χ(solvent). P0(solvent)**

Where,

**P(solution) **means the vapour pressure of the solution

**Χ(solvent) **means** **the mole fraction of the solvent

**P0(solvent)**means the vapour pressure of the pure solvent

We can understand this Raoult's Law by this example:

Let us consider a solution of volatile liquids A and B in a container. Both volatile liquids A and B are in the vapour phase. Hence, the vapour particles of both A and B exert partial pressure that contributes to the total pressure above the solution.

For Component A, that is,

**P1 = P1°x1 ………………(1)**

Where,

P1 is the Partial pressure of Component A

P° indicates the vapour pressure of Component A

x1 indicates the mole fraction of Component A

For Component B, that is,

**P2 = P2°x2 …………………….(2)**

Where,

P2 is the partial pressure of Component B

P° indicates the vapour pressure of Component B

x2 indicates the fraction of Component B

According to** Dalton’s law of partial pressures**, the total pressure [P(total)] of the solution in the container will be the sum of the partial pressures of both components in the solution.

P(total) = P1** + **P2

P(total) = P1°x1 + P2°x2

P(total) = P1°(1- x2) + P2°x2

**P(total) = P1°+(P2° - P1°)x2**

**Limitations of Raoult’s Law**

Raoult's law does not work perfectly under all conditions, there are some limitations attached to it-

- Raoult's law is only applicable to ideal gases.
- It must be examined if the solute dissociates in the solution. For instance, If NaCl (salt) is added to the solution it will dissociate into Na+ and Cl–.

**Deviation from Raoult's Law**

As we know, Raoult's law only works for ideal gases. If in a solution, there is no uniformity of attractive forces, i.e. adhesive forces(present between dissimilar molecules) and cohesion forces (present between similar molecules) then Raoult's law shows deviation-

**Negative Adhesion: **If the adhesion is stronger than cohesion then in such a condition it shows a negative deviation from Raoult's law as very few liquids get converted into vapour which further lowers the vapour pressure.

**Positive Adhesion: **If the adhesion is weaker than cohesion then in such a condition it shows a positive deviation from Raoult's law as the liquid particles escape the solution very easily increasing the vapour pressure.