2H + O -> H2O + energy
The mass of H2O is lower than the masses of the original components. Although extremelly low, chemical reactions have associated mass changes. No 'material' particles are created/destroyed, or emitted/captured in normal chemical reactions, but energy is (in the general case) radiated/absorved. This energy has an equivalent mass, and this is the change of mass related to chemical reactions (according to E=mc2). Since the mass change is very low, I think it is not possible to directly measure it, but I could be wrong.
Has somebody measured this change?