According to the law of conservation of mass, the number of iron atoms needed to complete the following reaction is fe + 3o2 → 2fe2o3

is 4

Explanation:

                      Fe + 3O₂ → 2Fe₂O₃

The law of conservation of mass states that "matter is neither created or destroyed in a system". Now, by careful inspection of the atoms reacting and the product formed, we see that the equation is not balanced.

Fe on reactant side is 1 but 4 on the product side. To balance it up, we simply put 4 as the coefficient behind the iron atoms.

                              4Fe + 3O₂ → 2Fe₂O₃

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