To balance a chemical reaction, it is important to remember that the number of atoms of each element in the reactants and the product side should be equal. This is to follows the law of conservation of mass where mass cannot be created or destroyed. So, the total mass that is used to react should have the same value of the total mass of the substances produced from the reactants. The balanced chemical reaction would be written as follows:
2h2 + o2 = 2h2o
Reactant = Product
H = 4 = 4
O = 2 = 2
Therefore, the correct coefficient for the hydrogen gas would be 2.
