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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 YA and YB respectively. Let PA and PB be the partial pressure of vapours A and B respectively and total pressure PT.
In Vapours phase:
YA= mole fraction of A in vapour phase
YB = mole fraction of B in vapour phase
                   (YA+YB =1)
In liquid solution phase:
XA = mole fraction of A in liquid phase
XB = mole fraction of B in liquid phase
                  (XA + XB = 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 XA­ and XB 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 PT is given by:
 Partial pressure of the gas = Total pressure x Mole fraction
                                        PA = PT YA and PB =PT YB
Where YA­ and YB 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/YA Vs 1/XA:
According to Dalton’s law of partial pressures, the total pressure PT is given by:
 Partial pressure of the gas = Total pressure x Mole fraction
Where YA­ and YB 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:

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