Assuming the reactants are balanced and we start with 5 carbon dioxide molecules and 10 water molecules, how many oxygen atoms would you expect to find in the products

Balance chemical equation for said reation is as follow,

                       CO₂  +  2 H₂O   →   CH₄  +  2 O₂

According to this equation,

1 Molecule ( 1 mole) of CO₂ reacts with 2 Molecules ( 2 moles) of H₂O to produce 1 Molecule ( 1 mole) of CH₄ and 2 Molecules ( 2 moles) of O₂.

So, if we take 5 molecules of CO₂ and 10 molecules of CH₄, it means we have multiplied the equation with 5. So, the equation will be,

                       5 CO₂  +  10 H₂O   →   5 CH₄  +  10 O₂

As there are 10 molecules of O₂ formed and each oxygen molecule contains 2 oxygen atoms, so the number of atoms in 190 molecules will be,

                                     = 10 × 2 

                                     =  20 Atoms of Oxygen