Consider the combustion of propane (C3H8) in the presence of oxygen:

Question

C3H8+5O2 => 3CO2+4H2O

How many grams of O2 are required to react completely with 3 moles of propane?

Answer 1

Using the coefficients in the balanced equation, convert 3 moles propane to moles oxygen.
3 moles C3H8 x (5 moles O2/1 mole C3H8) = ?? moles O2.
Then convert moles O2 to grams. g = moles x molar mass.