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Ideal Gas Law Deviations?

Discussion in 'MCAT Discussions' started by ilovelucy, Apr 18, 2007.

  1. ilovelucy

    ilovelucy This is Lucy. 2+ Year Member

    Mar 12, 2007
    Chicago, IL
    Does anyone have a good way to remember when ideal gases deviate from the ideal gas law?

    I was taught that you can always use PV=nRT with the caveat that when it is not ideal, the number you calculate using PV=nRT for a gas deviating is always slightly higher than the "real" number

    ideal>real. However, this doesn't always seem to work....

    Thanks in advance for the help!
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  3. djones473

    djones473 2+ Year Member

    Apr 14, 2007
    I think its easiest to understand why it happens and then you wont even really have to memorize anything.

    Two assumptions of the Ideal Gas Law are:

    1. Ideal gases take up no volume
    2. Ideal gases don't have intermolecular forces (no attraction)

    Real gases deviate at high pressures and low temperatures.

    Pressure: When pressure is moderately high, volume is less than predicted because there are intermolecular attractions so the molecules attract together and thus occupy less volume. At VERY high pressures (more than 500atm or so) the SIZE of the gas molecules becomes important and the volume would be greater. This is because at an infinitely high pressure the volume of the gas would approach zero (P and V are inversely related). Of course its not possible for the gases to take up no volume, so thus the ideal gas predicts a smaller volume than actual.

    Temperature: When the temperature is lower (close to condensation point) intermolecular forces play an important role and so the volume of the real gas will be less than predicted by ideal gas.

    So just remember: Intermolecular forces of attraction make the volume smaller than predicted. This attraction occurs at moderately high pressure (a few hundred atm) and low volume.


    The size of the molecules makes the REAL volume greater than the volume predicted by the ideal gas law at VERY HIGH pressures.

    hope that helps..
    ImDiene0412 likes this.
  4. The Force

    The Force 7+ Year Member

    Apr 14, 2007
    The ideal gas law assumes that the gas molecules have no volume. Since real gases have volumes, real volume is greater than ideal.

    It also assumes that there are no intermolecular forces between the gas molecules. Since real gases do have intermolecular interactions, molecules aren't as free to strike the walls of the container. Less molecules striking/and or less force per molecule striking the container leads to a pressure lower than ideal.

    Its a simplistic explanation but it should help.
  5. ilovelucy

    ilovelucy This is Lucy. 2+ Year Member

    Mar 12, 2007
    Chicago, IL
    Thanks to both of you -- both posts were super helpful!:)
  6. lifeistough


    Sep 10, 2005
    Five hundred atmospheres is a lot before it becomes a real gas. If it exceeds 10 atms, you are dealing under real gas standards. As user djones explained, under ideal conditions, the gas molecules are so spanned out from each other that volume can be negligible. Try picturing this instead of memorizing. Now take a piston and press down. The space of the gas molecules are closer together. That decrease in space, makes the gas molecules "count" and "noticeable". Before it was "Oh, that gas molecule over there, yes... that's it, that one! Oh don't worry about that small thing over there, look at all this space we have. So the tenet and the landowner signed on the lease that "gas molecules" were going to be negligible. Ten years went by (10 atms) and the tenet and land owner redrafted the lease because housing sizes were cut in half due to a decrease in space (there was an influx of population coming in). Now, this space is pretty valuable and those gas molecules are no longer negligible, they need to be "counted" in the lease. We have a reduction in space.

    Don't worry about memorizing if it's low or high temp. Just know that you have lots of "land" or volume and if you know that PV=nRT, there is a linear relationship, high V, high T. In opposite manner, high V, low P.

    This is a concept that is frequently tested. Please make sure you have covered all your tracks in this section. For clarity, when I mention that the gas molecules are no longer counted as negligible, that means they now have volume (>10 atm). On the MCAT, they will usually not place it at such a border line, but will give you enough signs that it is a real gas.

    Best Regards.

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