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Ideal Gas question

  1. Mar 25, 2013 #1

    CAF123

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    1. The problem statement, all variables and given/known data
    The ideal gas law states that 1 mole of an ideal gas at 0°C and 1 atmosphere=1.0125x105Pa (standard temperature and pressure, or “STP”) occupies a volume of 22.4litres (2.24x10-2m3).

    (i) Use this result to obtain an approximate value for the increase in volume that occurs when a mass of water at 100°C is converted into steam at the same temperature.

    For argon (monatomic gas, mass=40amu) at STP, estimate:
    (ii) The typical spacing between atoms
    (iii) The root-mean-square (rms) speed of the atoms


    3. The attempt at a solution

    i)I am not sure how to use the ideal gas law STP conditions. For one, this assumes 0 degrees temperature, while the question is concerned with 100 degrees. I know I have to incorporate the latent heat of vapourisation since the water is changing phase. (and at the same temperature)

    ii) Assuming small spherical atoms, ##r = \left(\frac{6}{\pi n}\right)^{1/3}##, where n = N_a/V, but I think this just takes me in a circle since I don't know V. Also, I think I have to assume ideal gas conditions, so each particle would be of negligible volume.

    Many thanks.
     
  2. jcsd
  3. Mar 25, 2013 #2
    Use the Ideal gas equation where the temperature is 373 K. I believe the volume occupied by water in liquid phase is very small compared to that in the gas phase and thus can be neglected in their difference.

    To find the spacing between the atoms, you just need to find the average volume occupied by an atom i.e. volume of gas per atom. Though the atom is considered infinitesimally small, you just need to find the volume of its immediate neighborhood and not that of the atom itself.
     
  4. Mar 25, 2013 #3

    SteamKing

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    In an ideal gas, what is the relationship between pressure, volume, and temperature?
     
  5. Mar 25, 2013 #4

    CAF123

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    So, when it says use this result, it just means use the ideal gas law? (PV = nRT). (So what I need is V where n=1, T = 373K, P = 1 atm, and simply solve for V?)

    So have I not to assume ideal gas conditions? (I.e particles take up negligible volume)

    This is what I did, and I derived the eqn in the OP. But n= N_a/V, so this only brings back in V, which is not what I want.
     
  6. Mar 25, 2013 #5

    SteamKing

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    The OP gives a volume for one mole of gas at STP.
     
  7. Mar 25, 2013 #6

    CAF123

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    Is this in response to my first question?
     
  8. Mar 25, 2013 #7

    CAF123

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    Volume per particle = V/N, V the volume of the container (assume 22.4 l) and ##N=N_A##. So the volume per particle is about 3.72 x 10-23 and so r is then about 2.3 x 10-16m from the relation given in the OP. Seems a bit small.
     
  9. Mar 25, 2013 #8

    Borek

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    Of what?

    No idea how you got it and what the "relation" from the first post is about. But assuming each atom occupies center of a cube of a volume that you already calculated, it is very easy to calculate the distance. And it is definitely different from the number you posted.
     
  10. Mar 25, 2013 #9

    CAF123

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    The 3.72 x 10-23l was the volume per particle that I have calculated.


    This is derived by assuming the atoms occupy a volume comprising a sphere.

    Assuming this, I get ##\approx 3.3 \times 10^{-8}m##.
     
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