During a chemical reaction, a fixed amount of iron combines completely with a fixed amount of oxygen to form iron oxide. Which statement best describes the mass of the iron oxide that is produced?

Explanation:

The mass of the iron oxide produced is the mass of the fixed amount of oxygen and fixed iron that reacted together.

This is predicated on the law of conservation of mass.

The law of conservation of mass states that "in a chemical reaction, matter is neither created nor destroyed, but atoms are rearranged".

The mass of the product is found by summing up the mass of the reactants.

So if we add the mass of oxygen and iron together, we get the mass of iron oxide that will be produced.