Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Average translational kinetic energy of nitrogen molecule

  1. Jan 10, 2008 #1
    1. The problem statement, all variables and given/known data

    2.0 moles of nitrogen (N2) gas are placed in a container whose volume is 8.4 10-3 m3. The pressure of the gas is 4.7 105 Pa. What is the average translational kinetic energy of a nitrogen molecule?

    average KEN2 = WebAssign will check your answer for the correct number of significant figures. J


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 10, 2008 #2

    Shooting Star

    User Avatar
    Homework Helper

    No equations, no attempt at solving?
     
  4. Jan 10, 2008 #3
    Attempt

    2.0 moles of nitrogen (N2) gas are placed in a container whose volume is 8.4e-3 m3. The pressure of the gas is 4.7e5 Pa. What is the average translational kinetic energy of a nitrogen molecule?

    So I used PV=nRT and solved for T, after solving for Temperature, I used KE=3/2kT and solved for the total kinetic energy. Now is this my final answer or are there more steps to this problem?
     
  5. Jan 10, 2008 #4

    olgranpappy

    User Avatar
    Homework Helper

    the *total* kinetic energy due to translational motion is 3/2 NkT

    you have solved for the kinetic energy of a typical molecule. which is what you want.
     
  6. Jan 11, 2008 #5

    Shooting Star

    User Avatar
    Homework Helper

    For a diatomic gas, you should use 5/2 KT.
     
  7. Jan 11, 2008 #6

    olgranpappy

    User Avatar
    Homework Helper

    mmm... no, he/she used PV=nRT to solve for the temperature which holds for any kind of ideal gas, be it composed of molecules or not... cf. Landau + Lif****z Stat Mech Volume I, section 42. then the translational kinetic energy is just 3/2 kT. cheers.
     
  8. Jan 12, 2008 #7

    Shooting Star

    User Avatar
    Homework Helper

    Quite right upto the point of finding T. But after finding T, he/she has used E_av = 3/2 KT, which will be true for point masses with only 3 degrees of freedom. The rotational energy is being neglected. So, E_av = 5/2 KT will give the correct answer.
     
  9. Jan 12, 2008 #8

    olgranpappy

    User Avatar
    Homework Helper

    but the question in the original post asks only for the average *translational* kinetic energy.
     
  10. Jan 12, 2008 #9

    Shooting Star

    User Avatar
    Homework Helper

    OK, I missed the "translational". In that case, it should be 3/2 KT. Thanks.
     
  11. Jan 12, 2011 #10
    Re: Problem

    and what is K? is that the same as nr?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook