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Light gas molecules vs heavy

  1. Aug 15, 2009 #1
    So, I was playing around with this virtual lab online. I could add light gas or heavy gas to try to make the piston move. I got the same results using both gases..in other words, it took the same amount of light gas to move the piston as it did using heavy gas. Why is that? One would think it would make a difference..
  2. jcsd
  3. Aug 15, 2009 #2


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    Nope. The parameters for the gas laws are temperature, pressure, volume and molarity. A mole is a mole, regardless of the gas; likewise volume. Temperature is a measure of the average kinetic energy of the gas particles--at first glance, it might seem that molar mass would make a difference here, but a heavy molecule with a given KE will move more slowly than a light molecule with the same KE. If you can find a simulation which tracks average particle velocity, you';ll see what I mean.
  4. Aug 15, 2009 #3

    ...that confused me because I thought the mass DIDN'T make a difference?
  5. Aug 15, 2009 #4
    Mass doesnt, becuase you are measuring the gas in terms of other variables that effectively take into account the mass.

    From ideal gas: PV=nRT

    As negitron was saying the Temperature is a way of describing the kinetic energy of the molecules. A for a heavy molecule to have the same KE as a light molecule it must be travelling slower.

    As pressure depends on the Temp (average energy) of the gas, we dont care about how much it weighs, only the energy it has.
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