# Simple mole fraction question

GreatEscapist
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.

## Homework Statement

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.

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## 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 don't know though, because I don't have the exact chemical formula.

Any tips or steps would be greatly appreciated. Thank you.

Puchinita5
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

Mentor
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.

Huh? Doesn't make any sense to me.

GreatEscapist: assume 100g of the mixture.

GreatEscapist
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.

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.