Answer:
By balancing the chemical equation
Explanation:
The Law of Conservation of Matter states that matter cannot be destroyed nor created.
That is, you must have the same amount of matter before and after a reaction.
Atoms are made of matter, so you must have the same number of each type of atom in the reactants as in the products. You must balance the equation.
Consider the reaction
2H₂ + O₂ ⟶ 2H₂O
You must have 2s in front of H₂ and H₂O to balance the atoms.
They give you four atoms of H and two atoms of O on each side of the arrow.
