An unused flashbulb contains magnesium and oxygen. After use, the contents are changed to magnesium oxide but the total mass does not change. What law explains these observations?

Law of conservation of mass explains these observations.

Explanation:

According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products formed must be equal to the mass of reactants taken.

In order to get the same mass on both sides, the atoms of each element must be balanced on both sides of the chemical equation.

Mass of reactants= mass of Mg+ mass of  = 48 g + 32 g = 80 g

Mass of products = mass of = 80 g

Thus mass of reactants is equal to mass of products, thus law of conservation of mass is obeyed.