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Simple Mole Conversions

  1. Nov 22, 2008 #1

    jacksonpeeble

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    What is the mass of methane (CH4) if it occupies 32.3 L at STP?

    We got a whole bunch of these problems on a worksheet with minimal instruction. If somebody could please aid me in solving this one, then I should be able to solve the rest.

    I assume that I must first find the masses of the individual components...

    C = 12.0107 * 1 = 12.0107
    +H = 1.00794 * 4 = 4.03176
    ---------------------------
    16.0483

    Then I honestly don't know what to do... I don't want the answer; I'd just really appreciate if somebody could show me how to do this. I tried searching for a guide online, but none of them really had what I was looking for.

    Thank you!
     
  2. jcsd
  3. Nov 22, 2008 #2

    symbolipoint

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    STP means Standard Temperature and Pressure.
    Use PV=nRT.
     
  4. Nov 23, 2008 #3

    Borek

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    Which - sadly - doesn't mean anything.

    jacksonpeeble: you have to know what temperature and pressure your teacher want's you to use as STP, as this thing is not precisly defined.
     
  5. Nov 23, 2008 #4

    jacksonpeeble

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    1 mole = 22.4 L at STP according to my teacher.

    What do I do with that equation? I assume that it is the same as (Pressure x Volume)/(Temperature x Moles) = R, but how is this applied?

    Thanks for the help so far, though!
     
  6. Nov 23, 2008 #5
    and then let 32.3 L = x mol at STP. you can get x. do you know how to do now? (forget that equation...for this problem, it is useless.)
     
  7. Nov 23, 2008 #6

    Borek

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    How many moles of methane then?

    What is molar mass of methane?

    While PV=NRT is the general method, that will give exactly the same results if applied correctly, you can solve the question using just the information quoted above.
     
  8. Nov 23, 2008 #7

    jacksonpeeble

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    Ah, so it's essentially just a ratio?

    So it would be 32.3/22.4 = 1.44 Moles?
     
  9. Nov 24, 2008 #8

    Borek

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    Now moles to mass and you are ready.

    Note, that if you will be given exact pressure and/or temperature value you will have to use PV=nRT equation.
     
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