Raoult’s law states that a solvent’s partial vapour pressure in a solution (or mixture) is equal or identical to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution.
Mathematically, Raoult’s law equation is written as:
Psolution = ΧsolventP0solvent
Where,
Psolution = vapour pressure of the solution
Χsolvent = mole fraction of the solvent
P0solvent = vapour pressure of the pure solvent
Further, we will understand the principle behind the law by looking at the example below.
Consider a solution of volatile liquids A and B in a container. Because A and B are both volatile, there would be both particles of A and B in the vapour phase.
Hence, the vapour particles of both A and B exert partial pressure, which contributes to the total pressure above the solution.
Raoult’s law further states that at equilibrium,
\(P_A = P^\circ _A x _A , P_B = P ^\circ _ B x_B\)
Where PA is the partial pressure of A.
P°A is the vapour pressure of pure A at that temperature.
xA is the mole fraction of A in the liquid phase.
Similarly, PB, P°B, xB
Hence,
