Ideal gas vs Real gas

Posts: 9
Joined: Wed Nov 22, 2017 9:59 am

Ideal gas vs Real gas

Postby simsimjoey » Sun Dec 10, 2017 1:06 am

Hi, I have a question about question 10 on chapter 22 from the review book.

The question asks, "compared to an ideal gas, a real gas would have..."

The answer here says b, which is smaller volume than predicted at lower temperatures and high pressure. Correct me if I'm wrong but as far as I know

Ideal gas model become inapplicable at high pressures and low temperatures
- High pressure : volume of particles no longer negligible —> volume of a real gas is larger than the expected ideal
- Low temperature : particles move slowly enough to interact —> volume of a real gas smaller than the expected ideal

They can have counteractive effect on volume, so shouldn't i use van der waal's equation if I were to solve this out...?

Thnx in advnace
Site Admin
Posts: 35
Joined: Tue Mar 01, 2016 10:19 am

Re: Ideal gas vs Real gas

Postby Administrator » Sun Dec 10, 2017 4:28 pm

Hi simsimjoey, thank you for posting your question!

You are correct: real gases deviate from ideal gas behavior at high pressures and low temperatures. At high pressures, the volume of real gases is larger than predicted, as ideal gases are assumed to have no volume while real gas molecules do have a small amount of volume. At low temperatures, the volume of real gases is smaller than predicted because the attractive intermolecular forces are no longer negligible when gas molecules move more slowly, and thus the distance between molecules decreases, whereas ideal gases are assumed to have no intermolecular interactions.

You're correct in that in such a case, we would have to use the van der Waal's equation to determine which would variable would have a greater effect, but you won't be asked to make that kind of calculation on the PCAT, and we would certainly need more information.

We apologize for any errors you may find in the BETA version of the PCAT Review Book. You can rest assured that we are working hard to make our content as error-free as possible before it goes to print. If you would like to provide feedback on any question, you are always welcome to use the Feedback button in your online Full Lengths or email us at

Good luck studying!

Return to “General Discussion”