Propane reacts with oxygen in a combustion reaction. What volume of carbon dioxide gas is formed when 20 L of propane are reacted with 55.7 L of oxygen?

Answer 1

C3H8 + 5O2 ==> 3CO2 + 4H2O
When gases are involved, we can omit the conversion to mols stage and use only volumes.
Convert 20L C3H8 to L CO2. 20 x (3 mol CO2/1 mol C3H8) = 60 L C3H8.
Convert 55.7 L O2 to CO2. 20 x (3 mols CO2/5 mols O2) = 33.4 L CO2.
One of these answers must be wrong; the correct value in limiting reagent problems is ALWAYS the smaller value and the reagent producing that value is the limiting reagent. Therefore, 33.4 L CO2 will be produced. There is some unreacted propane left over.