The mass of the product in a chemical reaction is equal to the total mass of the reactants. This fundamental principle is known as the Law of Conservation of Mass.

Here's why:

* Atoms are not created or destroyed in chemical reactions: They simply rearrange themselves into new combinations.

* The total number of atoms of each element remains the same: The mass of each atom is constant, so the total mass must also remain constant.

Example:

If you react 10 grams of sodium (Na) with 15 grams of chlorine (Cl), you will get 25 grams of sodium chloride (NaCl).

10 g Na + 15 g Cl = 25 g NaCl