In a chemical reaction, 300 grams of reactants are combined with 100 grams of reactant

b. both a and b react to completion. how much will the product weigh

I believe the mass of the product will be 400 grams. This is because according to the law of conservation of mass, mass is neither created nor destroyed. In a closed system, the mass of reactants is equal to mass of products. Therefore, since the total mass of the reactants is 400 grams then the mass of the products will still be 400 grams