Gas Stoichiometry Chemistry

Question

 A sample of propane (C₃H₈) gas was burned with excess oxygen gas. If 4.20 L of CO₂ were produced from the reaction of at STP, how many liters of propane were used in the reaction?

Answer 1

C₃H₈ + 5O₂ ==> 3CO₂ + 4H₂O ... balanced equation

Since there's no change in temperature and/or pressure, liters and moles are interchangeable. So ...

4.20 L CO₂ x 1 L C₃H₈ / 3 L CO₂ = 1.40 L C₃H₈

If you want to do it in a more conventional way, and also see that liters and moles are interchangeable, we can use the fact that at STP, 1 mole of an ideal gas = 22.4 L. Then we have ...

4.20 L CO₂ x 1 mol / 22.4 L = 0.1875 moles of CO₂ produced

0.1875 mol CO₂ x 1 mol C₃H₈ / 3 mols CO₂ = 0.0625 mols C₃H₈

0.0625 mols C₃H₈ x 22.4 L / mol = 1.40 L C₃H₈