DALTON'S LAW VERSES RAOULT'S LAW:

Determination of composition in vapour phase:

The composition of the vapour in equilibrium with the solution can be calculated applying Daltons’ law of partial pressures. Let the mole fractions of vapours A and B be Y_A and Y_B respectively. Let P_A and P_B be the partial pressure of vapours A and B respectively and total pressure P_T.

In Vapours phase:

Y_A= mole fraction of A in vapour phase

Y_B = mole fraction of B in vapour phase

                   (Y_A+Y_B =1)

In liquid solution phase:

X_A = mole fraction of A in liquid phase

X_B = mole fraction of B in liquid phase

                  (X_A + X_B = 1)

According to Raoult’s Law: The partial pressure of any volatile component of a solution at any temperature is equal to the vapour pressure of the pure component multiplied by the mole fraction of that component in the solution.

      Where X_A­ and X_B is the mole fraction of the component A and B in liquid phase respectively

According to Dalton’s Law:

The vapour behaves like an ideal gas, then according to Dalton’s law of partial pressures, the total pressure P_T is given by:

 Partial pressure of the gas = Total pressure x Mole fraction

                                        P_A = P_T Y_A and P_B =P_T Y_B

Where Y_A­ and Y_B is the mole fraction of the component A and B in gas phase respectively

Combination of Raoult’s and Dalton’s Law:

(3) Thus, in case of ideal solution the vapour phase is phase is richer with more volatile component i.e., the one having relatively greater vapour pressure

Graph Between 1/Y_A Vs 1/X_A:

According to Dalton’s law of partial pressures, the total pressure P_T is given by:

 Partial pressure of the gas = Total pressure x Mole fraction

Where Y_A­ and Y_B is the mole fraction of the component A and B in gas phase respectively

According to Raoult’s law:

On rearrangement of this equation we get a straight line equation: