As with any combustion reaction, the products of combusting a hydrocarbon fuel ( CxHy ) with oxygen (O2) are carbon dioxide (CO2) and water (H2O). A mass of 12.65 g for an unknown fuel was combusted in a reaction vessel containing an unknown amount of oxygen. At the end of the reaction, there still remained 12.61 g of the fuel as well as 0.0654 g of water and 0.1198 g of carbon dioxide. The oxygen was completely consumed during the reaction.

How many molecules of oxygen gas were initially present in the reaction vessel?