If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Check on Avogadro the balance of the water formation reaction:
2H2 + O2 → 2H2O
Build two H2 molecules and one O2 molecule. In properties (sequence Wiev → Properties), read the total mass of the written compounds.
On a new Avogadro page build a molecule of H2O and, in properties, read the value of the mass. You will notice that it is not equal to the one previously studied, but it will be exactly half of it. Add another water molecule and check in properties if the mass is now equivalent to that of the reactants of the balanced reaction.
Lavoisier’s law of conservation of mass on which the balance of reactions is based is thus verified.