# Homework Help: How does kinetic energy of gases relate to their volume?

1. Nov 14, 2012

Which of the following statements accounts for the observation that, at 300 K and 10 atm, the molar volume of C2H6(g) is less than that of H2(g)?

A C2H6 molecules are smaller than H2 molecules.
B The average kinetic energy of H2 molecules is greater than that of C2H6 molecules.
C The average kinetic energy of H2 molecules is less than that of C2H6 molecules.
D The intermolecular attractions in C2H6(g) are weaker than they are in H2(g).
*E The intermolecular attractions in C2H6(g) are stronger than they are in H2(g).

E is the correct answer, but it also seems like B is a reasonable answer, doesn't higher kinetic energy make a substance's volume increase? Isn't that why heating things causes them to expand?

2. Nov 14, 2012

### Simon Bridge

Average KE is proportional to temperature right?
T is the same for each sample, therefore...

Last edited: Nov 15, 2012
3. Nov 15, 2012

Oh, right. Thanks, idk how I missed that.

4. Nov 15, 2012

### Simon Bridge

Lots of people would - that's why the question is asked that way.
The idea is to reward you for reading with brain engaged (almost nobody does that anyway) and for understanding physics.

The "understanding physics" part gives you two possible answers - B and E (which is how you can tell you are doing well BTW) and you have to do a bit more work to figure out what makes one better than the other. I suspect the actual test is whether you realize the relationship between KE and T.

You can do it the other way around, by memorizing stuff, though... but it's harder.