1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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 kinetic energy

  1. Sep 15, 2012 #1
    1. The problem statement, all variables and given/known data

    Find the average kinetic energy in kJ/mol (including all degrees of freedom) of one mole of ethane at 410K.

    2. Relevant equations

    1/2Mu^s = kT/2
    u= avg. velocity
    N=degrees of freedom

    3. The attempt at a solution

    C2H6 degrees of freedom - 3n-6 = 18

    (18/2)(1.380x10^-23 j/k)(410K)= 5.09e^20J

    not sure how to get to KJ/mol from here.
  2. jcsd
  3. Sep 15, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    You should first check your arithmetics. Powers of 10, to be exact. Then comes the physics.
  4. Sep 16, 2012 #3
    Okay, thanks for the note on converting j to kj. My main question is how do I know how many degrees of freedom to use in order to complete the equation.
    My professor sent us an equation to use:

    the total internal
    energy is (3+J)/2RT where J is the number of rotational degrees of
    freedom in the molecule.

    I calculated the degrees of freedom to be 18 due to 3n-6. But that was not correct. Do I just use the rotational degrees of freedom or all degrees of freedom? Thanks.
  5. Sep 17, 2012 #4
    Got it! Thanks for the help
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook