Question

which best explains how the law of conservation of mass is present in a chemical equation?
o a. the number of atoms of each element making up reactants equals the number of atoms of the same elements making up the products.
o b. the mass of the atoms of a substance that are reactants equals the mass of the atoms of a substance that are products.
o c. the number of molecules of a substance that are reactants equals the number of molecules of the same substance that are products.
o d. the mass of a molecule of a substance that is a reactant equals the mass of a molecule of another substance that is a product.

Explanation:

The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. In a chemical equation, this is manifested by the number of atoms of each element in the reactants being equal to the number of atoms of the same element in the products. This is because atoms are just rearranged during a chemical reaction.

• Option B is incorrect because it refers to the mass of atoms of a single substance (reactant and product), but the law is about all elements in the reaction.
• Option C is wrong as the number of molecules of a substance can change (e.g., \(2H_2 + O_2

ightarrow 2H_2O\), number of \(H_2\) and \(O_2\) molecules vs \(H_2O\) molecules).

• Option D is incorrect as the mass of a reactant molecule and a product molecule (of different substances) is not necessarily equal.

Answer:

A. The number of atoms of each element making up reactants equals the number of atoms of the same elements making up the products.