<u>Answer:</u> The coefficient '3' should be placed infront of oxygen gas to balance the oxygen atoms.
<u>Explanation:</u>
Law of conservation of mass states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form.
This also means that total mass on the reactant side must be equal to the total mass on the product side.
A balanced chemical equation is defined as the equation in which total number of individual atoms on the reactant side is equal to the total number of individual atoms on product side.
For the given chemical reaction, the balanced equation follows:
By Stoichiometry of the reaction:
1 mole of ethanol reacts with 3 moles of oxygen gas to produce 2 moles of carbon dioxide gas and 3 moles of water molecule
Hence, the coefficient '3' should be placed infront of oxygen gas to balance the oxygen atoms.