**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**and_{A}**Y**respectively. Let_{B}**P**and_{A}**P**be the partial pressure of vapours A and B respectively and total pressure_{B}**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:

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