Law of conservation of mass states that If one or more substance undergo a reaction, then the mass of reactants and the products remains same.
CaO + CO2 ------------> CaCO3
Here mass of CaO is 56 amu and mass of CO2 is 44 amu, and mass of CaCO3 is 100 amu .
This shows that,
mass of reactants= mass of products
56 + 44= 100
i hope this is useful