What is the chemical equation for the burning of propane if all the products and reactants are in the gas phase?

Burning propane is a combustion reaction: propane reacts with oxygen to form carbon dioxide and water. Balance the atoms in C3H8 + O2 → CO2 + H2O: 3 carbon atoms require 3CO2, and 8 hydrogen atoms require 4H2O. Those products contain 10 oxygen atoms, so 5O2 is needed; choice B also shows every substance as a gas.

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