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Q on ideal gases

Discussion in 'MCAT Study Question Q&A' started by Deepa100, May 16, 2008.

  1. Deepa100

    Deepa100 Junior Member 10+ Year Member

    Aug 24, 2006
    Which of the following is NOT a criterion for a gas to be considered ideal?
    Correct Answer A
    A. The molecules have negligible mass.
    B. The molecules have negligible volume.
    C. The molecules do not experience intermolecular forces.
    D. The molecules do not liquefy at low temperature.

    I don't understand D. Can someone explain how it can be a characteristic of an ideal gas.
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  3. TypeSH07

    TypeSH07 Junior Member 7+ Year Member

    Mar 20, 2004
    The question says what is NOT a characteristic of an ideal gas so unless you asked your question incorrectly(asking why it is an ideal gas property) then you're right.
  4. Deepa100

    Deepa100 Junior Member 10+ Year Member

    Aug 24, 2006
    But the answer given by TPR is A which is also not a characteristic of ideal gas.
  5. MilkmanAl

    MilkmanAl Al the Ass Mod Physician 7+ Year Member

    Mar 23, 2008
    Kansas City, MO
    hSDN Alumni
    A liquid involves intermolecular forces between its constituent molecules. That's really all you need to know to rule out D, though there are lots of reasons an ideal gas can't be a liquid. Of course, shouldn't it just make sense that since you're talking about an idealized gas that it can't be a liquid and an ideal gas at the same time?
  6. Snowy

    Snowy Will I ever be a doctor? 2+ Year Member

    May 6, 2008
    My take: Definitely a tricky question. Most people would probably immediatly eliminate B and C. You may not have exactly remembered the ideal gas rules, which could mistakenly give you the idea that A isn't the answer. They follow four properties:
    1) no volume
    2) elastic collisions
    3) random motion
    4) no intermolecular forces

    So it's not necessary for molecules to be massless, which makes sense because they need mass to have kinetic energy (.5mv^2).

    As for D, maybe they wanted you to think that it can't be a gas if it is liquifing, which could be implied in ideal gas behavoir. Or maybe they wanted you to remember that real gases deviate from the ideal gas law at low temperatures and high pressures. Not sure though about the reasoning for D.

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