Gas Density

[EnginrTheFuture]

TBR Gen Chem: Ch. 7, Passage 11 , #76

Which of the following liquids would form the DENSEST vapor upon heating?

A. CH3CH2CH2OH
B. CH3(CH2)3CH2Cl
C. CH3(CH2)4CH2Br
D. (CH3)3CCH2OH

Given the passage is about real gasses.... wouldn't attractive forces of alcohols be dominant effect here? I know hydrogen bonding is more of a liquid/solid property but if the a value in van der waals equation is squared... why wouldn't H-bond like attractions be noticable? And how could we say that molecular mass (induced dipole/london dispersion effects) is more important?!

The answer is C because it has the highest molecular mass but I don't understand why molecular mass beats out H-bond attractions here.

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[milski]

The attraction forces matter only when it comes to determining the boiling point. Once you start talking about vapor, you are in the gas phase - the attraction between the molecule does not matter since they are really sparse now. At that point ideal gas law starts being a good approximation - the density will be proportional to the molecular weight of the molecule (at the same conditions - P&T).

 

[EnginrTheFuture]

aha... for some reason I took density to mean #molecules/volume (molar density).... not actual mass/volume.

I rarely ever see gasses referred to in anything but molar density (i.e. 22.4 L/ mol) so I guess they are referring to it that way here! Upon re-reading the answer key with your insight, the explanation is glaringly obvious/clear. Thanks for the wake up call