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Ideal gas vs Real gas
Kerolos_4875
#1 Posted : Wednesday, July 22, 2020 8:47:09 PM
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Hi,

Page 342 in the CC has a "Mcat tip" that states that at low P, PVreal < PVideal. How can both PV change? Wouldn't one increase and the other decrease? At least from what I have seen at looking at other resources that is what they state. Just to be clear, in an real gas, V > ideal and therefore Preal < Pideal since both PV cannot change in the same direction?
INSTR_Katerina_102
#2 Posted : Thursday, July 23, 2020 12:18:04 AM
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Hi Korolos,

When we use PV, it's more saying that the product of the two is changed.

For example saying PVreal < PVideal just means one of the two of P or V needed to have changed for this.

I would say that you are correct in pointing out that this is a relatively vague way of saying this, and it is better to say:

1. Intermolecular forces make measured pressures smaller than an ideal gas therefore Preal < Pideal, especially at lower Ts and Ps when molecules have more opportunity to interact.

2. Actual gas volumes make measured volumes larger than an ideal gas, and this becomes most noticeable at low V/ high P.

One thing I would contend with is that low V doesn't always come with low P (just a lot of the time) for example you can have fixed container volumes where P doesn't affect V.

I hope this helps!

Katt
Kerolos_4875
#3 Posted : Thursday, July 23, 2020 12:27:22 AM
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Thanks Katt!
INSTR_Katerina_102
#4 Posted : Thursday, July 23, 2020 12:33:48 AM
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No problem, it's a good question!
Kerolos_4875
#5 Posted : Friday, July 24, 2020 9:47:29 PM
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Hi! Sorry I am having a difficult time comprehending this especially since we are including things like "mod-high" and "very high." I'm trying to make it clearer but keep confusing myself. For the MCAT, is it safe just to say that real gases have a Preal < Pideal and Vreal > Videal? Sorry I can't seem to grasp this idea and looking at external sources has confused me even more.
INSTR_Katerina_102
#6 Posted : Saturday, July 25, 2020 4:19:45 PM
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Yep, it's totally okay to stick to that!
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