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Homework Help: Stat/Thermo Problem - Equilibrium Pressure in Permeable Sphere

  1. Mar 20, 2009 #1
    1. The problem statement, all variables and given/known data

    A sphere full of air at room temp and 1 atm is placed in a chamber filled with He gas at room temp and 1 atm. The sphere is permeable to He only. What will the equilibrium pressure in the sphere be?

    2. Relevant equations

    Po=initial pressure
    V=vol of sphere
    V'=vol of chamber
    Na=number of air particles
    Nh=number of He particles

    Ideal gas law: P=NRT/V

    I think that's all i need, but other eqs that might be relevant:

    S=ln(Q) : entropy=ln(number of states)
    dS/dE=1/T : derivative of entropy wrt Energy = 1/temp

    3. The attempt at a solution

    Here's what I think:

    since He is a noble gas, we can assume it does not interact with the air particles. Therefore, at equilibrium, a single He molecule is equally likely to be found anywhere in the chamber (V'), which means that the mean number of He particles in the sphere (V) will be V/V'*Nh.

    Then use the ideal gas law:

    can also solve for Nh and Na initially in terms of Po, R, T and V or V':

    Nh=PoV'/(RT), Na=PoV/(RT)

    and then plug in, rearrange and reduce:

    P=(Po/(RT))(V/V'*V'/V+V/V)*RT=Po(1+1)=2Po=2 atm

    So, is there anything wrong with what I did? I don't know what the answer should be, but I'm not confident in my answer because my prof mentioned that we should start by maximizing entropy.... which I could do, but this way seemed so much simpler. Suggestions? Any errors in my assumptions or approach? I don't find this material intuitive, so I could very well have made mistakes.

  2. jcsd
  3. Mar 21, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Your approach is somewhat limited because it can't easily accommodate complications such as a chemical reaction (for example). As long as you can also apply the entropy maximization approach, though, I don't see a problem. In fact, I think it's great that you're attacking problems from different angles.
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