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Homework Help: 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?
  2. jcsd
  3. Apr 10, 2010 #2


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    Homework Helper

    The answer is certainly rounded.

  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


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    Homework Helper

    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.

  6. Apr 11, 2010 #5
    Yea, its a multiple choice question.
    Thanks for your help!
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