Step-by-step explanation:
According to law of conservation of mass, mass can neither be created nor it can be destroyed. It can only be changed from one form to another.
Therefore, when 15 grams of reactants will combine together to result in the formation of a new compound then mass of the new substance formed will also be 15 grams.
Because, mass of reactants has converted into the mass of product.
Thus, we can conclude that 15 grams of product will in the new substance after the reaction.