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Gas Laws and Thermodynamics questions

  1. Oct 19, 2009 #1

    1) If air contains 1% AR by volume, what is the mass of Argon in a 2L balloon of air at 25 degress celsius?

    2) If for a real gas, P = RT(V - b) - a/V^2
    where a and b and R are constants, P = pressure, T = temperature and V = volume.
    What does a large a value represent in a real gas?

    3)A small pond has a layer of ice 1cm thick foating on its surface.
    (a) If the air temperature is -10 degrees C, find the rate in cm per hour at which ice is added to the layer. The thermal conductivity of ice is 0.592 W/mK.
    (b) How long does it take for a 20cm layer to build up?

    2. Relevant equations
    1) pV=nRT Ptotal = p1 + p2...pn

    3) H = k*A*dT/L

    3. The attempt at a solution
    1) I'm not sure if I'm supposed to use the simple addition equation where pressure of argon is just 1% of the total pressure.

    2) Obviously the gas would have lesser volume as a increases. But why do non-ideal gases exhibit this pressure drop?

    3) heat associated with freezing 1cm^3 water = mL = 0.001g * 334*10^3 = 334J
    That's really as far as I got...
    Last edited: Oct 20, 2009
  2. jcsd
  3. Oct 20, 2009 #2

    Andrew Mason

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    You have to use the fact that a mole of any gas occupies the same volume (if the temperature and pressure are the same). For example, one mole of any gas occupies 22.4 l at STP. This means that the volume occupied by a gas depends on the number of moles of gas ie. the number of freely moving gas molecules (or individual atoms in a monatomic gas such as Argon).

    If 1% of the volume is occupied by Argon, then what can you say about the number of Argon atoms compared to the number of other air molecules?

    I think you meant "lesser pressure as a increases". The pressure drop is due to the attraction between molecules - this requires energy to separate the molecules.

    The third question is not copied correctly. What is the rate of ice build-up?

  4. Oct 20, 2009 #3
    The rate in the third question isn't given. It's actually supposed to say "find the rate in cm per hour". My bad.
  5. Oct 20, 2009 #4

    Andrew Mason

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    Ok. The with that change the language now makes grammatical sense. But the question doesn't make sense:
    Why would ice build up at +10C?

  6. Oct 20, 2009 #5
    Again...my fault. It's supposed to be -10degrees. My bad.
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