Simple Mole Conversions

  1. jacksonpeeble

    jacksonpeeble 122
    Gold Member

    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

    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. symbolipoint

    symbolipoint 3,427
    Homework Helper
    Education Advisor
    Gold Member

    STP means Standard Temperature and Pressure.
    Use PV=nRT.
  4. Borek

    Staff: Mentor

    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. jacksonpeeble

    jacksonpeeble 122
    Gold Member

    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. 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. Borek

    Staff: Mentor

    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. jacksonpeeble

    jacksonpeeble 122
    Gold Member

    Ah, so it's essentially just a ratio?

    So it would be 32.3/22.4 = 1.44 Moles?
  9. Borek

    Staff: Mentor

    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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