In a chemical reaction, the law of conservation of mass states that the total mass of the reactants must equal the total mass of the products. In this case, the reactant is calcium carbonate (CaCO₃), and if the scientist starts with 30 grams of CaCO₃, then according to the law of conservation of mass, the total mass of the products (CO₂ and CaO) will also be 30 grams.
So the correct response is 30 grams.