Real Gas vs. Ideal Gas

[weanprednisone]

So B is the correct answer, because deviation from ideal gas law is that that gas should have lower to none IMF, and no volume. I is wrong because it actually has greater volume, and III is wrong because KE doesn't event have an effect on ideal gases with the absence of T and velocity.

Making a far stretch...
Videal=Vreal-nb
Pideal=Preal+n^2a/V^2
applying this to PV=nRT
(Preal+n^2a/V^2)*(Vreal-nb)=nRT<----real gas law.
Going back to choice I. CO2 exerts a greater pressure because its molecules have lesser volume.
If this question changed choice I. to CO2 exerts less pressure because it takes up a greater volume than He. The answer would be I and II only right? (not an actual choice) because when V increases, P has to decrease (in terms if T is constant) or can we make that assumption since it never mentioned about T...

But lmk if I'm wrong:
The concept of deviation from Ideal Gas Law
-No IMF=No attraction/repulsion between molecules
-No molecular volume, therefore no pressure exerted?

Thanks!

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[aldol16]

The V in the gas law does not refer to volume of individual particles but rather the volume of the container.

 

[weanprednisone]

The V in the gas law does not refer to volume of individual particles but rather the volume of the container.

Probably going to be a silly sounding question:
Then what does it mean when CO2 has lesser volume? is molecular volume not even related to the ideal or real gas law?

 

[NextStepTutor_3]

In the ideal gas law, "V" refers to the volume of the container. That's why P and V are inversely proportional. If you have a sample of CO2 in a 2-L container, and (while keeping all other factors constant) you shrink the container to a volume of 1 L, the pressure inside the container (exerted by CO2 particles on the walls of the vessel) will double.

Now, this means that the ideal gas law does NOT account for molecular volume. Why not? In short, this is the entire idea behind having an "ideal" gas law. One of the assumptions made for ideal gases is that the volume of their particles is negligible, which makes the ideal gas law nothing more than an approximation. This brings us back to the van der Waals equation (your real gas law). Instead of PV, the left side of this equation is (P + n^2a/V^2)(V - nb). In other words, our volume value is being "adjusted" by this "nb" amount. "nb" is what relates to the volume of the gas particles. A simple way to think about it is that V is the volume of the container, and "nb" is the volume taken up by the particles (where "n" = # of moles and "b" corresponds to volume / mol particles). V - nb, then, is the "volume of empty space" in the container. This is the value that, when reduced, leads to an increase in pressure.

Hope this helps!

 

[aldol16]

Probably going to be a silly sounding question:
Then what does it mean when CO2 has lesser volume? is molecular volume not even related to the ideal or real gas law?

CO2 does not actually have lesser volume - that's the point. It has a much larger vdW radius so it will occupy much more volume than He.

Molecular volume is not taken into account in the ideal gas laws - that's one of the critical assumptions you have to make for the ideal gas law to work. That is, you assume high temperatures and low pressures so that the gases are so far apart that their molecular volumes don't matter. So at high pressure and low temperature, this becomes more of a problem and that's why real gases diverge from ideal gases at the limits. So in a real gas, you should look at the vdW equation of state - that's probably the simplest correction for real gases. In that equation, you have a constant b term that's subtracted from the volume term. In other words, the ideal gas law underestimates the actual volume. Think about compressing an ideal gas to zero container volume. Conceptually, this is possible for an ideal gas because it occupies no volume. However, this is impossible for a real gas because at some point, you're going to start rubbing atoms onto one another and the Morse potential tells us we can't do that.