The vapor pressures of pure "X" and "Y" at 25 degrees celsius are 200mm and 400mm respectively

Question

anonymous · Answer

(a.) Which compound, X or Y, is more volatile?
(b.) which compound is present in highest concentration in the first few drops obtained in a fractional distillation?

kainui · Answer

What does volatile mean?

anonymous · Answer

@Kainui to my understanding it means that something can evaporate easily

kainui · Answer

Yeah, that's totally right. So if something can evaporate easily then that is just another way of saying it can turn from a liquid to a gas really quickly. Since it happens faster for more volatile compounds, there's more of it in the air creating a higher pressure the more volatile the compound is.

anonymous · Answer

@Kainui so for this question, how could we use that information to answer parts a & b

kainui · Answer

Well mm is one of several units of pressure, so the higher the number the higher the pressure. I have to go, good luck.

aaronq · Answer

i'm guessing that by saying "mm" you mean "mm Hg" which is the standard when using pressures based on column heights/lengths... which is unimportant to answering the question, but i thought you should think about this.

Vapour pressure can be (loosely) interpreted as concentration in a gas phase mixture, such that the component with a higher vapour pressure will have a larger population of gas molecules.

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B is more volatile thus has a higher VP and a higher concentration in the gas phase, when you condense the vapour, the mixture is now enriched in B.

SO for a) the more volatile component has a higher vapour pressure. 

For b), as you're heating up a liquid mixture, the composition of the vapour is dictated by the vapour pressure - the component with a higher VP will be of higher concentration in the vapour mixture.