The law of the conservation of mass states that in a closed system (totally closed to mass and energy), mass is neither created nor destroyed during a chemical reaction. We then expect that the total mass of the products should equal the total mass of the reactants. However, most chemical reactions performed in a typical school laboratory will not take place in a closed system.
To calculate the percentage difference between the mass of the reactants and the mass of the product relative to the mass of reactants, do the following:
% difference =