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