# Finding mole fraction of polymers from weight percents %

• Chemistry
watsup
Watsup! I'm taking this nano class which is awesome but I am getting stuck on the gen chem material! no bueno. just want to make sure i did this right. Thanks!

## Homework Statement

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"A blend contains 45wt% polystyrene and 55wt% polypropylene. What is the mole fraction of polystyrene in the blend?"

## 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

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.

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

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.