Refering to reaction 2 H2(g) + O2(g) -> 2 H2O(g) and proving that the law of conservation of mass is obeyed during a chemical reaction.

Answer 1

I don't see a question here.
In any balanced chemical reaction, the number of atoms of each element present is the same on the reactant and product sides. Furthermore, the mass of each atom is unchanged by the reaction. The total number of electrons also does not change. Therefore mass is conserved.