use atomic theory to explain why chemical reactions obey the law of conservation of mass

Answer 1

matter in the reactants just rearrange themselves into products, it is the same total matter.

Answer 2

Chemical reactions obey the law of conservation of mass because the total # of atoms of an element in a chemical reaction stays the same, which means that the atoms can't be created or destroyed because the total # of atoms always stays the same. It's Dalton's atomic theory!