Finding mole fraction of polymers from weight percents %

1. Sep 22, 2012

watsup

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
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45[g(PS)]/100[g(total)]
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. Sep 23, 2012

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.

3. Sep 23, 2012

watsup

Why wouldn't the molar mass for the polymer be the sum of the molar masses of the two different monomers?

4. Sep 23, 2012

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.

5. Sep 23, 2012

chemisttree

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.