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Homework Help: A storage Tank at STP

  1. Jul 12, 2014 #1
    1. The problem statement, all variables and given/known data[/B]

    a Storage tank at STP contains 18,5kg of nitrogen (N2). a) What is the volume of the tank? b) What is the pressure if an additional 15.kg of nitrogen is added without changing the temperature.

    2. Relevant equations
    n= m/molar mass
    P= nRT/V

    3. The attempt at a solution

    For a) I got n= m/molar mass --> 18.5 X 1000 / 28 = 660.71 mol
    1 mole occupies 22.4 L
    --> V= 660.71 X 22.4 = 14 799.99 L 14.8 m^3

    For b) I have T= 273.15 n= (18.5 +15) x 1000 /28= 1196 mol
    i think for some reason V is assumed to be constant and doesnt change so ,
    --> P= nRT/V, wich gives me 183.44 N/m^2

    However in b the answerr is suppose to be 1.83 X10^5 Pa, or 183 X 10^ 3 Pa, ans I thought n/m^2 is the same units as Pa. I would like clarification, thanks
  2. jcsd
  3. Jul 13, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    "for some reason"??
    Don't you know?!
    i.e. does the tank change size?

    So you are out by a factor of 1000?

    Take a closer look at your numbers. Do a dimensional analysis to check the units.
    i.e. show me the exact calculation - you missed a term or divided by 1000 when you shouldn;t have or forgot an exponent.

    Note: the calculation is easier is you compare the state equations before and after the extra mass is added.
  4. Jul 14, 2014 #3
    Oh I see, I should have used V in m^3 not in L. Thank you
  5. Jul 14, 2014 #4
    Regarding Part b: From the ideal gas law, if the temperature and volume are held constant, the pressure is proportional to the number of moles.

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