The simple answer is because of the law of conservation of mass. This law states that matter can not be created or destroyed only transfered or rearranged. You can't have 5 oxygens reacting and only 3 oxygens as the product. What happened to the other two oxygens? They couldn't have magically disappeared. How much you put into an equation is what you will get out of the equation. This is one if the guarantees of life.
Hope this helped!
~Just a girl in love with Shawn Mendes
