Find the volume of oxygen when burning 15 liters of propane.

The molecular formula of propane is C3H8. Let’s compose the reaction equation for the combustion of propane:
C3H8 + 5O2 = 3CO2 + 4H2O;

Let’s find the amount of substance in propane:
15 / 22.4 = 0.67 mol;

There is a factor 5 times higher before oxygen than before propane. This means that the amount of substance in oxygen is 3.35 mol. Let’s find the volume of oxygen:
3.35 * 22.4 = 75.04 liters;

Answer: To burn 15 liters of propane, you need 75.04 liters of oxygen.