Which equation obeys the law of conservation of mass?

A.

2Fe + 3O2 → 2Fe2O3

B.

4Fe + O2 → 2Fe2O3

C.

Fe + O2 → Fe2O2

D.

4Fe + 3O2 → 2Fe2O3

D. remember the number of all the atoms of each molecule must stay the same. if 4 Fe is given the 4Fe must come out. you can know this because in the products side, there is a 2 after the Fe meaning that there are 2 Fe bonded together, with 3 O, the 2 before the Fe indicates that there are 2 of this specific molecule. which makes 4 Fe, meaning that we need 4 Fe for this equation to produce such a molecule. same thing for the Oxygen. there are 3 O in each of the 2 molecules in the products side meaning that we would need 6 O in the reactants side to make such an equation