Balancing equations requires the use of coefficients. In reality, it takes two molecules of hydrogen peroxide for this reaction to happen, and it produces two molecules of water and one molecule of oxygen. The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. The mass of the products in 2 H2 + O2 = 2 H2O equal the mass of the reactants!! I hope this answers your question to the full extent! <3