Consider the combustion of propane (C3H8) in the presence of oxygen: C3H8+5O2 => 3CO2+4H2O How many grams of O2 are required to react completely with 3 moles of propane?

Answer 1

Looks like it takes 5 moles of O2 to react with 1 mole of propane.
So you will need 15 moles of O2.
convert that to grams.