Question
Sun January 30, 2011 By: Doctor Srishty

the vapour pressure of a solvent decreased by 10mm in two columns of mercury when a non volatile solute was added to the solvent.the mole fraction of the solute in the solution is 0.2.what should be the mole fraction of the solvent if the decrease in the vapour pressure is to be 20mm of mercury

Expert Reply
Tue February 22, 2011

The equation governing the change in the vapor pressure when you use a nonvolatile liquid or solid is just a special case of Raoult's Law, Ptot = PaXa + PbXb. Since the second component has a negligible vapor pressure (nonvolatile), Raoult's law reduces to

P = P0X
where P is the vapor pressure of the mixture, P0 is the vapor pressure of the pure solvent and X is the mole fraction of the solvent.
Mole fraction of solute = 0.2
so mole fraction of solvent = 0.8 
So
P-10 = 0.8(P)
So P= 10/0.2 = 50
 
In second case
50-20 = x.50
30/50= x
So mole fraction of solvent = 0.6
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