To substantiate the assertion of mass conservation in Stephanie's experiment, one would need to demonstrate that the total mass of the reactants equals the total mass of the products after the reaction. This principle, known as the Law of Conservation of Mass, states that mass cannot be created or destroyed in a chemical reaction.
In this case, Stephanie combines 10.0 grams of zinc with 25.0 grams of bromine. The total mass of the reactants is:
\[ \text{Total mass of reactants} = \text{mass of zinc} + \text{mass of bromine} = 10.0 , \text{g} + 25.0 , \text{g} = 35.0 , \text{g} \]
After the reaction, Stephanie should measure the mass of the products (in this case, zinc bromide). If the mass of the products is also measured to be 35.0 grams, this result most strongly substantiates her assertion of mass conservation.
Therefore, the statement that would MOST strongly substantiate her assertion of mass conservation is:
"The total mass of the products after the reaction equals 35.0 grams, the same as the total mass of the reactants."