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Homework Help: Ideal Gas Density!

  1. Nov 12, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the density of nitrogen gas (molecular mass = 28 u) at a pressure of 1.7 atmospheres and a temperature of 332 K?

    2. Relevant equations
    You must convert pressure from atmospheres to Pascals. I'vedone this using the conversion 1 atmosphere = 1.01*10^5 pascals.
    I was also told that I needed to convert molecular mass into kilograms. Given a molecular mass of 28u. I calculated the mass in kilograms to be 0.00002799 kg/mole of Nitrogen.

    3. The attempt at a solution

    1 molecule of Nitrogen = 28u
    28u * 1.066*10^-27=4.648*10^-26 grams per molecule.
    1 mole = 6.022*10^23 molecules therefore 1 mole = 0.027990256 grams.
    There are 1000 grams in 1 kg. So 1 mole = 0.00002799 kg of Nitrogen.
    If I assume 1 mole and am given R=8.31 t=332k and p=1.7 atmospheres
    My equation should come to be volume = (1)(8.31)(332)/171700.

    This gave me a volume of approximately 0.01607

    If I then divide my mass in kilograms by this I get 0.00175 as my density which is wrong.

    Am I missing a step?

    Am I using the wrong equations?

    Thanks in advance for any help you can offer!
  2. jcsd
  3. Nov 12, 2007 #2


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    Science Advisor
    Homework Helper

    1 atmoshere is 101.4 KPa ( a useful number to remember)
    You have the mass wrong, 1 mole of a substance is it's atomic mass in grams.
    So N2 = 2*14 = 28g/mole or 0.028 kg/mole.
    Don't work in individual atoms, work in moles - makes the numbers more reasonable.

    Then you need PV = nRT to tell you how many moles are in a given volume at a given temp/pressure.
    See http://en.wikipedia.org/wiki/Pv=nrt for values / units.

    Hint - another useful number to know is that 1 mole of a gas at stp is 22.4 litres, this comes out of the above equation.
  4. Nov 5, 2010 #3
    A quick question-

    "Hint - another useful number to know is that 1 mole of a gas at stp is 22.4 litres, this comes out of the above equation."

    ...Then what is the volume of 1 kilogram mole of an ideal gas?
  5. Nov 5, 2010 #4


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    Staff: Mentor

    What is kilogram mole? How is it related to just a mole?

    Note that STP is ambiguous and in different contexts means different things.
  6. Nov 5, 2010 #5
    kilogram mole is supposed to be the amount of a substance containing as many elementary particles as there is in 12kg C12 isotope.
  7. Nov 9, 2010 #6
    If 1 gram mole of an ideal gas has a volume of 22.4 metre cube,then perhaps 1 kg-mole has a volume of 22.4x10^3 metre cube??
  8. Nov 9, 2010 #7


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    Staff: Mentor

    Yes and no.

    Yes - your thinking is correct. No - 22.4 cubic meters is a wrong value to start with.
  9. Nov 10, 2010 #8
    Then I guess the volume of 1 gram mole is 22.4 litres and that of 1 kg-mole is 22.4 metre-cube??
  10. Nov 11, 2010 #9


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    Staff: Mentor

  11. Nov 11, 2010 #10
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