[sn1sn2e1e2]

Hello again, another quick question:

A tank of propane contains both a liquid and gas phase. Which is true?

I. 2 phases have the same potential energy but different kinetic energies.

II. 2 phases have the same molar masses but different densities.

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I chose II as the only correct one (which is right) but I am not sure about the reasoning for I.

Ok, I know that liquids have more potential energy than solids (since you have to put in energy) but I am not sure about the relationship between GASES and LIQUIDS.

According to some websites (not yahoo), gases do not have any PE but I am not sure. Do gases have more potential energy than liquids, thus proving statement I false?

Thankss

[Borek]

Hello again, another quick question:

A tank of propane contains both a liquid and gas phase. Which is true?

I. 2 phases have the same potential energy but different kinetic energies.

Kinetic (and potential) energies are extensive properties. Answer as given makes no sense without additional information about how much of each phase is present.

Even then it would be still ambiguous, as potential energy is always measured with regard to some reference point - not knowing what is the reference point it is impossible to tell what is the potential energy.

Chance are you mean "potential/kinetic energy per molecule", which would be an intensive property. That still leaves us with the undefined reference point.

[sn1sn2e1e2]

The question did not mention molecules. Let me rewrite the whole question:

A partially filled tank of propane contains both a liquid and a gas phase, Which of these statements about the contents of the two phases are correct? (USNCO 2009)

I. The two phases have the same potential energy but different kinetic energies.

II. The two phases have the same molar masses but different densities.

II is right since moles "cannot change" and liquid molecules travel slower than gas molecules.

In I, gas molecules definitely have more KE than liquid molecules but I am not sure about the "potential energy". Is there an explanation for this, perhaps?