Question

When comparing two molecular substances, the one with the higher vapor pressure at 25 degrees C must have?
a) higher boiling point
b) lower heat of vaporization
c) stronger intermolecular attractions
d) the smaller molar mass
e) the greater polarity

i don't get how to approach this. Please explain

Answer 1

@aaronq Can you please explain this to me, please

Answer 2

you kind of have to make sense out of each option; remember that a higher vapour pressure means that if you have two different substances in different containers, the one with a higher vapour pressure will have more molecules in the gas phase than the one with the lower VP.

The answer is b) lower heat of vaporization

it means that it takes less energy to take a molecule from the liquid to the gas phase.

Answer 3

i can go over why the rest are not correct tomorrow because i'm about to log off. Just keep this open, take care.

Answer 4

I will leave it open. Thank you so much! @aaronq

Answer 5

So i'll go briefly, step by step, but first you should know that VP is the pressure exerted by a gas on the walls of a closed container.



\(\sf\large P=\dfrac{Force}{Area}\)

a) higher BP

The one with a higher boiling point will have less molecules in the gas phase (because the energy necessary to volatilize them is greater) so the one with a lower BP will have more molecules in the gas phase, hence a higher VP.

b) lower heat of vaporization

Heat (or enthalpy) of vaporization is the energy necessary to volatilize (put molecules from liquid phase into gas phase) a given number of molecules at their BP. A sample with a lower vap enthalpy will require less energy to do this. Hence more molecules will be in the gas phase, higher VP.


c) stronger intermolecular attractions

Higher IMFs, higher BP (see a))

d) the smaller molar mass

This one can be deceiving; if you compare 2 hydrocarbons the one with a higher molar mass will have a lower BP, hence a higher VP. BUT the question does not indicate that this is the only possibility. Stronger IMF's (e.g. hydrogen bonding) could be present in a smaller molecule (vs a hydrocarbon) and thus a higher BP could be observed for a lower molecular weight compound. (i.e. compare BP's of butane and water).

e) the greater polarity

Greater polarity -> stronger IMF's (dipole-dipole attractions), which means a higher BP (see a)).