The law of conservation of mass states that. “The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another”. According to the law of conservation of mass, the mass of the reactants must be equal to the mass of the products for a low energy thermodynamic process.
I’m pretty sure that when balancing a chemical equation no one is thinking about the conservation of mass. It’s all about counting atoms. When you have the same number of each kind of atom on either side of the arrow, then the equation is balanced. Now, that’s not to dismiss the law of conservation of mass which is verified upon closer inspection of the balanced equation.
eg CH4 + O2 → CO2 + H2O - needs balancing.
you use the idea that each type of atoim is conserved - so looking at this you can see it is not currently correct. On the LHS there are 2 O atoms and on the RHS there are 3 O atoms- the number of atoms must remain unchanged. We dont bother about their mass ( although mass is conserved)’