We also have six CO2 molecules each with two oxygen atoms in it, so we have 12 oxygen atoms in those molecules.
So we have a total of 18 oxygen atoms on the left side.
You need a different mole equation to work with gases.
Stoichiometry with gases everything stays the same but the equation you use to find the moles.
Stoichiometry is the base for all modern chemistry.
The method I'm going to use is like a flow chart opposed to the "railroad tracks" that is taught by most teachers and books. The definition of combustion is a fuel when burned with oxygen produces only H2O and CO2.
We have to balance the molecules to have the same number of atoms.
6 H2O- Since we have 12 hydrogen atoms and each molecule has 2 atoms we 6 molecules of water.
Now multiple the number of moles by the coefficient of substance you find the moles of and divide it by the coefficient of the substance you are translating it into. You can now find find the mass of the product produced using the Sm/Mw=n equation. You can use the law of conservation of mass to do this.
Mass on both sides of the equation must be the same.