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)
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)
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)
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.