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This question already has an answer here:

Does chemical energy contribute to the mass of an object? I don't mean the bond energy, but the possible energy that could be released (i.e. Does an atom of oxygen and a molecule of hydrogen (H2) have more mass together (even when not bonded) than the sum of the masses separately (assuming each one is considered in empty space)?)

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marked as duplicate by Floris, John Duffield, ACuriousMind, Bill N, Qmechanic Oct 6 '15 at 23:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ The answer "yes" is too short to count as a valid answer on this site. What are you looking for, really? This link perhaps? How is "the energy that could be released" really different than the bond energy? $\endgroup$ – Floris Oct 6 '15 at 19:50
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    $\begingroup$ Possible duplicate of Conversion of mass to energy in chemical/nuclear reactions $\endgroup$ – Floris Oct 6 '15 at 19:51
  • $\begingroup$ But let's say the oxygen atom is in an empty universe with no H2, it would have no "chemical potential energy" as there is no possible lower energy state. $\endgroup$ – DOS4004 Oct 6 '15 at 19:53
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    $\begingroup$ @Floris: that does settle the question then. 'Chemical energy' is no more than a inaccurate term for bound energies of molecular orbitals. The mass lost in chemical reactions is almost imperceptibly small though, so Lavoisier was almost, almost, almost (repeat many times), almost right! ;-) $\endgroup$ – Gert Oct 6 '15 at 21:33
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    $\begingroup$ @Floris: "A 100 years for a mind like that to come around and 5 mins to chop his head off!" someone apparently claimed... $\endgroup$ – Gert Oct 6 '15 at 21:39
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I believe I now understand the question - and my earlier comment and the link for a potential duplicate does not apply.

If I'm right, you are actually asking this question:

If you take two containers - one with hydrogen, and one with oxygen - and you weigh them separately; then allow the gases to mix, will the mass of the mixture be different?

If the atoms react, the chemical energy releases will result in a corresponding loss of mass for the combined system; see for example this earlier question or the answer to another related question.

If the atoms did not react, there is no change in the chemical energy; but interestingly, there is a change in the entropy of the system. And entropy is closely related to energy. Speculating here - my thermodynamics is very very rusty:

<speculation>

In fact, if you have a perfectly reversible engine, you can do work by changing the amount of entropy: $\Delta U = T\Delta S$. This means that the small change in entropy as you mix your gases will result in a small change in energy, and this should produce a change in mass.

</speculation>

I'm willing to be proven wrong on this last point. As I said - thermodynamics rusty.

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  • $\begingroup$ Thanks! I realized that even when separated, the energy of the hydrogen or the oxygen remains the same. However, it would be inaccessible. $\endgroup$ – DOS4004 Oct 6 '15 at 22:43
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One example of saying "yes" is that the mass of a single atom is different when the same atom is bonded by chemical reaction to another atom. In this case, a little bit of mass is converted to chemical energy for bonding atoms in the compound.

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  • $\begingroup$ i'm talking about unbonded atoms considered together $\endgroup$ – DOS4004 Oct 6 '15 at 20:12
  • $\begingroup$ I should reference to this post. $\endgroup$ – X Qi Oct 6 '15 at 20:28

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