[AirForceOne]

In this question:

Which substance in each pair has the higher vapor pressure at a given temperature?
(b) Cl2 or Br2
(c) CH3Br or CH3Cl

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How do I do this? My teacher told us that since Cl2 and Br2 are non-polar, you have to use molar mass? And that since CH3Br and CH3Cl are polar, you would have to use the electronegativity trend? I don't really understand, I know how to identify polar/nonpolar, but why do you have to use molar mass and electronegativity in the different situations?

Thanks!

[swap]

Hi

I was thinking about a similar question several days ago and now I have an idea but I don't know if it's true.I think you know about Van der Waals' forces.

So you should use molar mass for non-polar molecules.Cl is about 35,5 g/mol and Br is about 80g/mol if I remember right.That means Van der Waals' forces of Br2 are stronger than Van der Waals' forces of Cl2.That also means vapor pressure of Cl2 is higher than Br2.

Well, polar molecules...You know that kind of molecules are dipoles and that creates a force between molecules.The molecules pull other molecules.And you should use electronegativity for polar molecules.Because when the difference gets bigger you can say that the molecule is more polar.
Electronegativity of Bromine:2.96
Electronegativity of Chlorine:3.16
As you see Cl is more electronegative then Br.That means dipol moment of CH3Cl is bigger than dipol moment CH3Br.So the force between CH3Cl molecules is bigger than the force between CH3Br molecules.That means vapor pressure of CH3Br is bigger.

As you can see I told about different active forces for polar molecules and non-polar molecules.That's why you have to use molar mass and electronegativity in the these situations.

I'll be glad if I can help you.Please correct if there is a mistake...

[DmytriE]

I agree with the second part but the non-polar molecules wouldn't you have to do something with the rms?  The larger the molecules the greater the momentum.  Wouldn't this cause the Br2 to easily evaporate, vaporize?  I get the definition of vapor pressure confused very easily.