$H_2O$ vs $2H^+ +O^{2-}$ I am wondering the mass deficit between 2*H + O and H2O.
I.e. if I would know the accurate mass of one atom of hydrogen on it's own and the mass of one atom of oxygen on it's own how much of a difference would there be between the mass 2 times the mass of a single hydrogen atom plus one times the mass of a single oxygen atom and the actual atomic mass of the reacted H2O?
 A: When water is formed by burning hydrogen, energy is released. So a molecule of water is slightly lighter than two atoms of hydrogen plus one atom of oxygen. We can estimate the mass difference by using $E=mc^2$ on the enthalpy of formation of water.
From the Active Thermochemical Tables of the Argonne National laboratory, we find that the enthalpy of formation of water at STP is 285.8261 kJ/mol.
To get the mass deficit for a single molecule of water, we divide that by Avogadro's number and by $c^2$, expressing the result in atomic mass units. Google Calculator is handy for things like this, since it has lots of constants and units built in. Using this query in the Google search bar:
(( 285.8261 kJ/mol)/(avogadro constant))/c^2 in daltons

we obtain $3.180244 \times10^{-9}$ atomic mass units. 
This isn't the exact mass-energy requested in the question, since it assumes the hydrogen & oxygen are in their usual diatomic molecular form, not isolated atoms. But it's a reasonably good approximation. ;)
