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!

Calculating rms speed of hydrogen molecules

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

    The rms speed of the molecules in 1.1 g of hydrogen gas is 1800 m/s.

    500 J of work are done to compress the gas while, in the same process, 2000 J of heat energy are transferred from the gas to the environment. Afterward, what is the rms speed of the molecules?

    I've already solved for the total translational energy(1800 J) before more work was done to the molecules. The thermal energy was also 3000 J.

    2. Relevant equations

    E = (5/3)(1/2)mv^2



    3. The attempt at a solution

    Since 500 J are compressed, and 2000 J are released, the energy would then be 300 J. I tried setting this equal to the above equation, but all answers I entered were wrong.
     
  2. jcsd
  3. Mar 16, 2012 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    300 J as final energy is wrong.
    When compressing, work is done on the gas, increasing the internal energy. So 500 J is added, 2000 J heat removed.

    ehild
     
  4. Mar 16, 2012 #3
    1800(Initial Energy) + 500 - 2000 = 300 J

    Am I missing something or does the thermal energy also play a factor.
     
  5. Mar 16, 2012 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    The work done and the heat removed changes the thermal energy, which is 5/2 RT for one mole of a diatomic molecule.


    ehild
     
  6. Mar 16, 2012 #5
    I took that approach and my answer was still wrong.
     
  7. Mar 16, 2012 #6

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Could you show your work in detail?

    ehild
     
  8. Mar 16, 2012 #7
    3000 J(thermal energyinitial) + 500 J - 2000 = E thermalfinal = 5/3(1/2)mv2
     
  9. Mar 17, 2012 #8

    ehild

    User Avatar
    Homework Helper
    Gold Member

    3000 J was the initial thermal energy . It decreased by 1500 J, the final thermal energy is 1500 J. The thermal energy is 5/3 (1/2 mv^2). So how much is 1/2 mv^2, the translational kinetic energy? How much is v, the rms speed?


    ehild
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Calculating rms speed of hydrogen molecules
Loading...