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RMS speed of gas

  1. Dec 9, 2016 #1
    1. The problem statement, all variables and given/known data


    (introduction course of university physics)


    A 5.0- liter gas tank holds 1.7 moles of monatomic helium (He) and 1.10 mole of diatomic oxygen

    (O2), at a temperature of 260 K. The ATOMIC masses of helium and oxygen are 4.0 g/mol and

    16.0 g/mol, respectively. What is the ratio of the root- mean- square (thermal) speed of helium to

    that of oxygen?

    Answer: C

    2. Relevant equations
    V(rms) = sqrt(3kT/m)

    3. The attempt at a solution
    This is the only equation I found from my book about rms speed of gas, but the variables seems unrelated at all. I have no idea on how to do it.

    Please tell me how to deal with it in a more detail way.
    Thank you very much.
     
  2. jcsd
  3. Dec 9, 2016 #2

    DrClaude

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    Staff: Mentor

    Unrelated to what? This is actually the correct equation to use.
     
  4. Dec 9, 2016 #3
    but how to pluck in the numbers? what should i manipulate to get the required variables?
    can you be more specific please?
    thanks
     
  5. Dec 9, 2016 #4

    DrClaude

    User Avatar

    Staff: Mentor

    The question is
    Start with that. How do you write that ratio?

    What are k, T, and m?
     
  6. Dec 9, 2016 #5
    just divide the two which substituded in the equation

    of course I know I have to find out all the variables mentioned in the question and the equation in order to answer the question...
    but I just have no idea how to figure them out...
    can you be more specific? i can't even guess them out... please don't just simply ask me to calculate the answer, because that's exactly my problem for can't getting the answer!

    k is boltzmann constant
    t is temperature
    m is mass of a molecule
    of course i know them, by just substitute them!, but how to find them out?

    sqrt(3*1.38e-23*260/1)/sqrt((3*1.38e-23*260/16)=4
    but the answer is 2.8

    I REALLY have no idea
    whats wrong by plucking them in
    or if i did something totally in the wrong track

    please be more specific on how to do!!
    like how you figure out the answer
    or your steps to calculate it
    i can't do anything further based on your question which that seems to be common sense

    thank you very much
     
    Last edited: Dec 9, 2016
  7. Dec 10, 2016 #6

    ehild

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    Homework Helper
    Gold Member

    What are the masses of the molecules? The atomic mass of He is not 1 and the molar mass of the O2 is not 16.
     
  8. Dec 10, 2016 #7
    oh yes thanks
     
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