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Homework Help: Kinetic Theory Problem

  1. Jan 6, 2010 #1
    1. The problem statement, all variables and given/known data

    At standard temperature and pressure (273 K and 1 atmosphere), oxygen has a mass density () of 1.43 kg/m3. The molecular mass (m) of oxygen is 32 u. Calculate the number (n) of molecules in one cm3 of oxygen. (Round to three significant figures.)



    2. Relevant equations

    R = 8.31 J/molK
    kB = 1.38times10^-23 J/K


    3.

    I thought I had to use the following formula.. n=p(density not pressure)V/m.. When i put it in. it didnt get the right answer

    PLEASE HELP
     
  2. jcsd
  3. Jan 6, 2010 #2

    ideasrule

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    You have 1.43 kg of oxygen molecules and each molecule weighs 32 u. How many molecules do you have?

    There are this many molecules in 1 m^3.
     
  4. Jan 6, 2010 #3

    cepheid

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    Welcome to PF.

    YOu need to show your calculation steps explicitly so that we can try to figure out where you went wrong.
     
  5. Jan 6, 2010 #4
    Do I have to convert u into kg??
     
  6. Jan 6, 2010 #5
    the "u" is whats throwing me off. I've never seen that before
     
  7. Jan 6, 2010 #6

    ideasrule

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    You don't have to, but I'm curious about how you're going to divide if you don't.

    "u" is the atomic mass unit. Use the Internet and check it up!
     
  8. Jan 6, 2010 #7

    cepheid

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    Why would your teacher ask you question about something that he hadn't taught or defined in class? It stands to reason that you are supposed to know what it means.

    Start here:

    http://en.wikipedia.org/wiki/Molecular_mass

    read the first paragraph until you get to "unified atomic mass unit", which should be a hyperlink. Then you can clink on that link to find out what it is.
     
  9. Jan 6, 2010 #8
    Hey i'm right there with you. He doesnt do anything. but give me a sec.
     
  10. Jan 6, 2010 #9
    Do i have... 2.69x10^25 molecules??

    Sorry I'm a total physics idiot.
     
  11. Jan 6, 2010 #10

    cepheid

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    I thought you already knew what the right answer was? (Otherwise, in your first post, how would you have known that your first attempt was wrong?)
     
  12. Jan 6, 2010 #11
    I am using webassign so when I input the wrong answer it tells me so.
     
  13. Jan 6, 2010 #12

    ideasrule

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    In one m^3, yes, but the question wants the answer in cm^3.
     
  14. Jan 6, 2010 #13
    So i multiply that answer by 1x10^-6 and that should give me the right answer ?
     
  15. Jan 6, 2010 #14

    ideasrule

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    yes, that's right
     
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