The iron burns in oxygen to form iron tetroxide. Does that follow the law of conservation of mass? So why do you want fe+o2 to ignite fe3o4? The iron burns in oxygen to form iron tetroxide. Does that follow the law of conservation of mass? So why does fe+o2 ignite fe3o4?

Any chemical reaction follows the law of conservation of mass.
You're not right to write that. An iron trioxide needs three iron atoms and four oxygen atoms. You' re writing an iron atom. It's not right.