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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: oompaloo on October 27, 2005, 07:42:24 PM

Title: vapor composition of hexane and pentane solution
Post by: oompaloo on October 27, 2005, 07:42:24 PM

Does anyone know how to figure out the vapor composition of an 80% hexane and 20% pentane solution when the vapor is condensed and the liquid below is heated?  Thanks in advance!

Title: Re:vapor composition of hexane and pentane solution
Post by: mike on October 27, 2005, 08:08:18 PM

P_solvent = X_solvent x P^o_solvent   [Raoult's Law]

X = mole fraction of solvent
P = vapour pressure of pure solvent

P_total = P_solvent1 + P_solvent2 [Dalton's Law]

P_total = X_solvent1 x P^o_solvent1 + (1 - X_solvent1) x P^o_solvent2

These equations might help (?)

Title: Re:vapor composition of hexane and pentane solution
Post by: oompaloo on October 27, 2005, 08:45:09 PM

I have those formulas but I have no idea where to find the numbers to plug into those equations???  The only information I have is a solution containing 80% hexane and 20% pentane.  How do you figure out the numbers to plug into the equations???

Title: Re:vapor composition of hexane and pentane solution
Post by: mike on October 27, 2005, 08:51:20 PM

I would have thought you could work out the partial pressure of hexane and pentane for 80% and 20% (0.8 and 0.2 mole fraction) and then the partial pressures would be proportional to the amount of vapour.

Title: Re:vapor composition of hexane and pentane solution
Post by: mike on October 27, 2005, 09:04:04 PM

For example:

mole fraction of solvent 1 in vapour is:

P₁/P_total

P₁/P₁+P₂ = X_solvent1xP^o_solvent1/X_solvent1 x P^o_solvent1 + (1 - X_solvent1) x P^o_solvent2

 :) :)