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Homework Help: Thermodynamics: two pistons; different pressures, volumes, and temperatures

  1. Mar 22, 2012 #1
    1. The problem statement, all variables and given/known data


    Two thermally insulated vessels are connected by a narrow tube fitted with a valve that is initially closed as shown in the figure. One vessel of volume V1 = 15.2 L, contains oxygen at a temperature of T1 = 280 K and a pressure of P1 = 1.77 atm. The other vessel of volume V2 = 23.0 L contains oxygen at a temperature of T2 = 460 K and a pressure of P2 = 2.35 atm. When the valve is opened, the gases in the two vessels mix and the temperature and pressure become uniform throughout.

    What is the final temperature?

    What is the final pressure?

    2. Relevant equations
    I really do not know which are relevant

    PiVi/ Ti = PfVf/Tf

    1 atm = 101325 pa

    1L = 1000cm3

    ΔE internal = Q (isovolumetric process)

    ΔE internal = W (adiabatic process)

    ΔE internal = Q + W (first law of thermodynamics)

    3. The attempt at a solution
    I really do not know where to start or what formula to use so any hints would be helpful. We didn't have very much time to cover all this at all (very watered down) so if there is something I am supposed to give and don't give, then sorry.

    The way I see it, I think you would solve for the tube as if it were another vessel / piston, but I am not sure how to even do that :(

    P1 = 179345 pa
    V1 = 152 m3
    T1 = 280 kelvins

    P2 = 238113 pa
    V2 = 230 m3
    T2 = 460 kelvins
    Last edited: Mar 22, 2012
  2. jcsd
  3. Mar 22, 2012 #2


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    as the system is isolated ,its internal energy will remain constant.
    oxygen is diatomic, so internal energy of of n moles =2.5nRT=2.5PV
    let final pressure be P,then

    initial moles of oxygen=(P1V1)/RT1 +(P2V2)/RT2
    so,P*(V1+V2)=(initial moles)*R*T (T is final temp.)
    solve to get,T.
  4. Mar 22, 2012 #3
    Thank you so much. I got them both now even though you practically did the formula for me.

    I knew that it was an isolated system too but didn't see how that would help either but...

    Don't know where you got this from.

    And for temperature, I didn't realize you could use the ideal gas law like that (adding the pressure / volumes of the cylinders equal to find n).

  5. Mar 23, 2012 #4


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    oxygen is diatomic ,so it has 3 translational and 2 rotational degrees of freedom.
    so,internal energy per mole of oxygen is (3+2)/2 *RT ...(equipartition of energy theorem)
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