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Finding mole fraction of polymers from weight percents %

  1. Sep 22, 2012 #1
    Watsup! I'm taking this nano class which is awesome but Im getting stuck on the gen chem material! no bueno. just wanna make sure i did this right. Thanks!

    1. The problem statement, all variables and given/known data[/B]
    "A blend contains 45wt% polystyrene and 55wt% polypropylene. What is the mole fraction of polystyrene in the blend?"

    3. The attempt at a solution
    I think I remember having to change the percents into grams,
    then find the molar masses of the total blend and the polystyrene.
    Then divide the grams by the molar masses leaving moles/moles --> so unitless

    Polystyrene: (12*8 + 1*8) = 104[g/mol]
    Polypropylene: (12*3 + 6*1) = 42[g/mol]
    total= 146[g/mol]

    (45/104)/(100/146) ≈ 0.632
  2. jcsd
  3. Sep 23, 2012 #2


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    Staff: Mentor

    This is at least strange. Unless they mean something else than they asked, to calculate molar fraction you need to know molar mass - of the polymer, not of the monomer.

    Unless for some reason they decided to use a definition of molar fraction that is different from the definition used in general chemistry.
  4. Sep 23, 2012 #3
    Why wouldn't the molar mass for the polymer be the sum of the molar masses of the two different monomers?
  5. Sep 23, 2012 #4


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    Staff: Mentor

    Molar mass depends on how many monomers reacted when producing one polymer molecule. And it is (almost) never a strict value, it is an average.
  6. Sep 23, 2012 #5


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    Science Advisor
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    Gold Member

    You are using 100g of polymer in your calculation. You have determined that 45g of this is PS. Where you went wrong is with the factor 100/146. You assume here that the molecular wt of the polymer is the algebraic sum of the molecular weights of the individual monomers. This would be true only for a polymer derived from an equimolar mixture of the two monomers.
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