Let two volatile liquids A and B dissolve each other to form an ideal solution. Then the vapours above the solution will contain the vapours of A and B.

Applying Dalton’s law of partial pressure the total vapour pressure of the solution will be

Psolution = PA + PB

Where PA and PB are the partial vapour pressures of A and B.

Solution being dilute (Condition for ideality) *Raoult's law can be applied.*

*Raoult’s* law for binary solutions state that *“the vapour pressure of any volatile constituent of a binary solution at any given temperature is equal to the product of the vapour pressure of pure constituent and its mole fraction”*.

PA = PºA XA

PB = PºB XB

PºA and PºB are vapour pressures of pure A and B respectively.

cA and cB are mole fraction of A and B in liquid solution respectively

Hence

Ps =

Hence,

Ps =

This equation is of the form y = mx + c

The value of m (slope) may be (+ve) or (–ve) depending upon whether PB0 > PA0 (m = + ve) or

PB0 < PA0 (m = –ve).

Hence a plot of a graph of Ps versus XB will be a straight line with an intercept on y-axis equal to pA and slope equal to

Vapour composition means to find out the mole fraction of A and B in vapour. Dalton’s law of particle pressure is used to calculate the vapour composition, as we know

Partial pressure = mole fraction × total pressure

PA =

or

Similarly for B

where and are mole fraction of ‘A’ and ‘B’ in vapour phase and

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