The Law of conservation of Energy applies to mass as mass is a form of energy, E=mc2.
it conserves mass
The Law of Conservation of Energy states: Energy cannot be created or destroyed but merely change from one form to another.Einstein showed that matter is a form of energy. When a nuclear decay occurs...
The Conservation of Matter Law states that matter can neither be created nor destroyed, just changed in form.
go to wikipedia.org and search "The Law of Conservation of Mass" you"ll find everything you need yhere.