As with any combustion reaction, the products of combusting a hydrocarbon fuel (CxHyCxHy) with oxygen (O2O2) are carbon dioxide (CO2CO2) and water (H2OH2O). A mass of 13.49 gg 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 13.47 gg of the fuel as well as 0.0327 gg of water and 0.0599 gg of carbon dioxide. The oxygen was completely consumed during the reaction. How many molecules of oxygen gas were initially present in the reaction vessel? Express the number of molecules to three significant digits.

When a reversible reaction is occurring and the pressure is increased then the equilibrium which is dynamic will shift in order to facilitate this change by virtue of Le Chatelier's principle.  When pressure is increased, the equilibrium shifts to favor the side with the most number of moles.

For example, the theoretical formation of ammonia is characterized by the reaction:   N₂   +   3H₂    ⇆   2NH₃ 
If the pressure is increased in this reaction system, then the equilibrium would shift to the left, since that side has three moles of hydrogen and one mole of nitrogen for a total of four moles while the right side only has two moles.