Mass is never lost or gained in chemical reactions. It is said that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning. This is because no atoms are created or destroyed during chemical reactions.
The principle of conservation of mass allows you to work out the mass of one substance in a reaction if the masses of the other substances are known.
In practice, it is not always possible to get the entire calculated amount of product from a reaction. This is because: