How does a balanced chemical equation show the law of conservation of mass?

Answer 1

If it's balanced, you have the same number of atoms of every element on both sides. So if you add up the atomic masses, you will have the same answer on both sides.

That's all there is to it. Don't let anyone kid you on that there is any mystery here.

Answer 2

if you add 20 atom masses and take 2 squared its really not balanced