Lavoisier's law

Lavoisier's law, also known as the law of conservation of mass, states the following:

In a chemical reaction, the total mass of the reactants equals the total mass of the reaction products

Example: In the formation of water from hydrogen and oxygen (2H2+O2→2H2O ), 4 grams of hydrogen reacting with 32 grams of oxygen will produce exactly 36 grams of water. The number of h- and N-atoms is equal before and after the reaction. The mass is thus preserved