Question

Identify the mass law that each observation demonstrates. a flashbulb contains magnesium and oxygen before use and magnesium oxide afterward, but its mass does not change.

Answer 1

The mass law is LAW OF CONSERVATION OF MASS.
The law of conservation of mass states that mass, in an isolated system is neither created nor destroy by chemical reaction or physical transformations. 
This implies that, the mass of the chemicals before the reaction will be equal to the mass of the product after the reaction.