Choice A: 2 moles of H2 and 1 mole of O2
Choice B: 2 moles of H2O
Next question:
You put 2 moles of H2 and 1 mole of O2 in a sealed container and send a jolt of electricity in. The two gases react and produce 2 moles of H2O. Does the mass of the container change?
Last question:
Does the mass of an airplane change after the passengers eat their meals?
No, this is the last question: Why should we do your homework for you?
I think this is meant to be a comment on Cecil’s column regarding your last question, in which case it goes in a different forum.
As for the first and second questions, IANAChemist, and I don’t know the bonding energies of those things offhand. Whichever has more energy in the bonds masses more, since the matter is the same.
As for the third: Of course the mass doesn’t change. Nothing’s entered or exited the system as a result of eating the meals. If anything it gets slightly lighter because of radiating energy into the colder surrounding air, and burning off jet fuel, but those have nothing to do with the meal.
To within the precision to which they can be measured with any experimental instrument, 2 moles of H[sub]2[/sub] plus one mole of O[sub]2[/sub] will weigh exactly the same as 2 moles of H[sub]2[/sub]O. Assuming equal temperatures in both cases, the H[sub]2[/sub]O will in fact weigh slightly less, but the difference would be undetectably small. In your example of putting the gasses in a sealed chamber and sparking them, there would not even be that small difference of mass, provided you’re asking about the moment immediately after the combustion before any heat had a chance to escape: The lesser binding energy would be exactly compensated for by the greater thermal energy.