Rust forms when iron metal reacts with oxygen gas. Which chemical reaction best supports the law of conservation of mass?
A. Fe + O2 --> Fe2O3
B. 2Fe + O2 --> 2Fe2O3
C. 3Fe + 202 --> Fe2O3
D. 4Fe + 302 --> 2Fe2O3

D. 4Fe + 302 --> 2Fe2O3

Explanation:

The equation that bests supports the law of conservation of mass is the choice D;

       4Fe  +   3O₂  →   2Fe₂O₃

Based on the law of conservation of mass, the number of atoms on both sides of the expression must be the same. Only the fourth choice proves this law;

           4Fe  +   3O₂  →   2Fe₂O₃

                                                Number of atoms

    Elements              Left hand side                            Right hand side

        Fe                                4                                                     4

        O                                 6                                                      6

We can see that both side of the expression have the same number of atoms and the equation is balanced.