[Dimitri1337]

Hi, I just joined this forum and I'm very excited about it.  It looks to be well kept. 

Anyway, I have a comprehensive exam coming up in a few days.  I was going over the pretest my teacher gave me when I began having trouble with this.  The question:  Aspirin has 60% C, 4.48% H, and 35.5% O.  It has a molecular mass of 180 g/mo.  What are its empirical and molecular formulas? 

I started forgetting the process one I converted everything to moles.  I had 5 moles of Carbon, 4.48 moles of Hydrogen, and 2.219 moles of Oxygen.  What process do you guys find most effective and efficient when the number of moles isn't a number that nicely divides into the others?

I would ask my teacher, but unfortunately he's gone right now for an unknown reason.  Tomorrow we'll have a substitute who knows nothing about chemistry.  The next day is my final. 

Your help would be greatly appreciated!

[Borek]

What is 35.5% of 180? How many oxygen atoms is it?

[Dimitri1337]

63.9 grams.  This gives us 3.99375 moles of oxygen which converts to 2.404 x 10^24 atoms of oxygen.  I do not see where this is going. 

[Dimitri1337]

I've figured my problem know.  The numbers where just giving me a brain fart.  I ended up getting C9H8O4 for the empirical formula and the same for the formula mass.  Thanks for your taking your time to help me even though i'm not so sure where you you going with it.  In fact, could you elaborate on what you were trying to explain earlier?  I'm pretty curious.

[Borek]

You missed forest for the trees. If molar mass is 180 g/mol, mass of a single molecule is 180 amu - that means 64 amu for oxygen, or four atoms of oxygen per molecule.