The Law of Conservation of Mass
Established in 1789 by French Chemist Antoine Lavoisier
States that mass is neither created nor destroyed in any ordinary chemical reaction.
Or more simply, the mass of substances produced (products) by a chemical reaction is always equal to the mass of the reacting substances (reactants).
E.g., 1.00g carbon + 5.34g sulphur ? 6.34g carbon disulphide
2.00g carbon + 10.68g sulphur ? 12.68g carbon disulphide