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!

Translational Energy of Molecules and Vrms

  1. Apr 9, 2010 #1
    1. A quantity of molecular hydrogen (H2) gas fills a one liter container at a temperature of 200 K and pressure of 1 atm.

    What is the average (rms) speed of the molecules?


    2. Relevant equations

    KE_trans = 1/2 M<v^2>
    Energy_trans = 3/2kT = 3/2RT
    H2 = 2g/mol

    3. The attempt at a solution

    Assuming 1 mol of H2:
    mass = .002kg

    1/2 M<v^2> = 3/2RT
    (.002kg)<v^2> = 3R(200K)
    <v^2> = 2494200m/s
    Vrms = sqrt(<v^2>) = 1579.30m/s

    Which is wrong.
    The correct answer is 1600m/s. (Not a rounding error.)

    Can anyone help me?
    Thanks!
     
  2. jcsd
  3. Apr 10, 2010 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    The answer is certainly rounded.

    ehild
     
  4. Apr 10, 2010 #3
    The reason I dont believe its rounded is because the options are:
    (a) 16 m/s
    (b) 74 m/s
    (c) 274 m/s
    (d) 1600 m/s
    (e) 4570 m/s

    So if it were rounded it would be rounded to 1580 and in my
    physics exams they always give you the exact answer or really close to it.
     
  5. Apr 10, 2010 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    You result is correct. Do you need to present your calculation, or just have to choose one value? If it is a multi-choice question mark the closest one.

    ehild
     
  6. Apr 11, 2010 #5
    Yea, its a multiple choice question.
    Thanks for your help!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook