The Law of Conservation of Mass states that matter is neither created nor destroyed in chemical reactions. The total mass of reactants must equal the total mass of products. This was partially demonstrated in the previous video by balancing equations and counting atoms.

To prove the law of conservation of mass through calculation, we use atomic mass. Atomic mass is the mass of an atom, typically found with decimal values in the periodic table. For simplicity, rounded-off values will be used in calculations to compare masses of reactants and products.

Using a balanced equation (4Al + 3O2 → 2Al2O3), the video demonstrates calculating the total mass of reactants and products. With aluminum's atomic mass at 27 and oxygen's at 16, the total mass for both reactants and products is found to be 204 grams, proving mass conservation.

For the reaction 2Na + Cl2 → 2NaCl, the atomic masses used are sodium (Na) at 23 and chlorine (Cl) at 35.5. Calculating the masses, both reactants and products yield 117 grams, further confirming the law of conservation of mass.

The final example uses the equation 2Fe2O3 + 3C → 4Fe + 3CO2. Atomic masses are iron (Fe) at 56, oxygen (O) at 16, and carbon (C) at 12. The calculation shows that both sides of the equation result in 356 grams, validating that the equation is balanced and follows the law of conservation of mass.