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Homework Help: Pressure Question

  1. Jan 31, 2005 #1
    I know this question is easy, but for some reason I am unable to figure it out.

    The first part of the problem wants to know what the volume is in a pressurized tank. I figured it and it equals 2.1 ft^3.

    The second part asks for the specific volume at 70 F and 200 psi.
    So far all I have is v(specific volume) = V/m. Or 1/d = v

    The gas in the tank is air, so basically I need to find the mass of air @ 70 F and 200 psi. I have the volume, so that is not the problem.

    The answer to the question is .98 ft^3/lbm

    Should I be using ideal gas laws? If I do use the ideal, I'm assuming I need to convert units. I'm having a problem converting psi to lbf/ft^2...

    I can't come up with this answer, so if anyone can give me a hand I would appreciate it.


    Frustr8ed, as the name states. :confused:
  2. jcsd
  3. Jan 31, 2005 #2

    Andrew Mason

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    Homework Helper

    It would help if you would state the whole question. Use the ideal gas equation of state, PV=nRT to find n (number of moles). The answer won't be in ft^3/lbm.

  4. Feb 1, 2005 #3


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    You may want to consider using this form of the ideal gas law:

    [tex] P \nu = RT [/tex]


    [tex]P = Pressure (Lb_{f}/ft^2)[/tex]
    [tex]\nu = Specific Volume (ft^3/Lb_{m})[/tex]
    [tex]R = Universal Gas Constant (for air: 53.35 Ft*Lb_{f}/°R*LB_{m})[/tex]
    [tex]T = Temperature (°R) [/tex]
  5. Feb 1, 2005 #4
    Thanks for the responses. I believe that last equation is the one I need to use, but I was having trouble converting units.

    Thanks Again,

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