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Homework Help: Simple mole fraction question

  1. Sep 25, 2012 #1
    This is for my chemical engineering class...for some reason, I am struggling with this problem. I'm not quite sure how to connect the dots..I feel like I know how to do this, though.

    1. The problem statement, all variables and given/known data
    The mass fraction of benzene in a benzene-toluene mixture is 0.450. What is the mole fraction of toluene in the mixture? Molecular weights of benzene and toluene are 78.11 and 92.12 respectively.

    2. Relevant equations

    3. The attempt at a solution

    Mass fraction of toluene is .550

    Now, that is dimensionless, and I don't know how to get going from here. I know it's simple, but I've forgotten my general chemistry...
    I was thinking I just multiply that mass fraction by the molecular weight of toluene, but then that's still in g/mol...then I thought about adding them, which would make more sense (since I have the mass fraction). I dunno though, because I don't have the exact chemical formula.

    Any tips or steps would be greatly appreciated. Thank you.
  2. jcsd
  3. Sep 26, 2012 #2
    well mass fraction is of benzene is mass_benzene/(mass_mixture)=.450

    they give you the molecular weights of each component, and you can find the molecular weight of the mixture by just adding the molecular weights.

    since the masses are in grams, and moelcular weight is in g/mol, if you divide the mass by molecular weight you get just moles. just make sure what you do to one side of the equation you do to the other.

    And then just manipulate the equation the same way you did to get the mass fraction of toluene.

    Hope that helps
  4. Sep 26, 2012 #3


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    Staff: Mentor

    Huh? Doesn't make any sense to me.

    GreatEscapist: assume 100g of the mixture.
  5. Sep 26, 2012 #4
    Aha- I knew I forgot something simple. Thank you for the 100 g suggestion. I didn't know if assuming an amount of grams was a legal move.
  6. Sep 26, 2012 #5


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    Staff: Mentor

    Technically you should assume mass m and solve with symbols, you would find in the end m cancels out. But 100g is much faster and less cumbersome.
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