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Some problems with mole fraction

  1. Oct 24, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-10-24_23-34-53.png

    2. Relevant equations
    not sure

    3. The attempt at a solution
    First, I'm a bit confused as to what the mole fraction of the products mean. Can someone explain? Is it something to do about the total number of carbon and hydrogen atoms in methane, ethene and propene?

    Just can't figure this out.
     
  2. jcsd
  3. Oct 24, 2015 #2

    Borek

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    Staff: Mentor

    Mole fraction is

    [tex]\frac {number~of~moles~of~the~substance}{total~number~of~moles~of~all~substances~present}[/tex]
     
  4. Oct 24, 2015 #3
    Could you give me a hint as how this information can be used in finding X? I have no clue.
     
  5. Oct 25, 2015 #4

    Borek

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    Staff: Mentor

    To be honest, I have no clue either. The only way to solve is to make some false assumptions about how the cracking process works - that is, to assume it is possible tow rite a simple reaction equation. That is not correct - stoichiometry of cracking is rather messy and doesn't follow simple equations, overall reaction equation will contain plenty of fractional coefficients - not because it follows some other chemistry rules, but because overall process is a combination of many different parallel reactions going at the same time and competing. Plus, it is not clear if the mole fraction takes into account produced hydrogen or not (if there are alkenes produced, hydrogen must be present between products, yet it is not listed).

    I think I could be able to prove each of the answers to be the right one. Futile exercise.
     
  6. Oct 25, 2015 #5
    Oh I see. Well the answer is B, if it helps any. I've been trying to see how the mole fraction given is used to get that answer.
     
  7. Oct 25, 2015 #6

    Borek

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    Staff: Mentor

    If they think the answer is B they probably mean the cracking goes like

    X → methane + 2 ethene + propene

    so X contains 1 +2*2+3 = 8 atoms of carbon.

    But if the reaction is

    X → 2 methane + 3 ethene + propene

    X contains 2*1 + 3*2 + 3 = 11 atoms of carbon and the answer is D.
     
  8. Oct 25, 2015 #7
    Oh....ok. Thanks for your time.
     
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