We are accounting both for the powdered iron and the oxygen it reacted with
Work Step by Step
When powdered iron is left exposed to the air, it reacts with oxygen in the air, rusting. The mass of the rust is greater than the mass of the powdered iron because the oxygen in the air now adds mass to the iron. The law of conservation of mass states that mass is conserved in all chemical changes. Therefore, when we determine the mass of the rust, we are accounting both for the powdered iron and the oxygen it reacted with.