How is mass conserved in chemical reactions?
The same way as for any equality: i.e. equivalence with respect to mass on left hand and right hand sides. And this reflects the fact that mass is conserved
I am not quite clear as to your question, but I am certainly willing to modify this answer. You know that mass is conserved in every chemical reaction. If I start with 10 g of reactant (from all sources), AT MOST I can get 10 g of product.
Hydrocarbon combustion reactions clearly illustrate the principle, mind you, most of the time the oxygen reactant is unmeasured, and we do not measure its mass;
If some (say one molecule) of the product were carbon monoxide, or even soot, how would we modify this equation so it is stoichiometrically balanced?
Under controlled conditions, certainly we could measure the mass of the hexanes, and of the oxygen reactant. These would precisely equal the mass of the water and carbon dioxide products. Why? Because the atoms themselves have definite masses, and chemical change always conserves mass.