Chemistry
posted by Tabetha .
I'm using the equation c3H8 + 5O2 yields 3 CO2 + 4H2O + 2220 kJ to figure out how many moles of O2 will be needed to produce 3450 kJ of the above reaction. I know this involves a proportion, but I have no idea what to write. Can you show me this proportion and I'll figure it out? Thanks for your explanation.

3450/2220=Moles/5
solve for moles O2. Study the proportion, and make certain you know why it is correct. 
Is the answer 7.7 moles then?

I get 7.770 mols O2 and since you are allowed 4 significant figures I would use all of them.