Putting a Q on the left hand side would be an error. Q indicates that the energy is not bound in the molecular configurations. If you measure the mass of a qu

Putting a Q on the left hand side would be an error. Q indicates that the energy is not bound in the molecular configurations. If you measure the mass of a quantity of water accurately enough, the mass will be lower than the mass of the equivalent amount of hydrogen and oxygen gas. The difference would be so small that our most sensitive instruments would not be able to detect it. Likewise, in a nuclear reaction, the only thing changing is the nuclear binding energies of the protons and neutrons. However, since the energy involved is many orders of magnitude higher, our instruments are sensitive enough to detect the difference in mass.