# Which has Greater Mass **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [July 4, 2006, 4:48pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174 "2006-07-04T16:48:39Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![smile4me2222](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@smile4me2222](https://boards.straightdope.com/u/smile4me2222)\ **Post date:** [July 4, 2006, 4:48pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174/1 "2006-07-04T16:48:39Z")
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? ---
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [July 4, 2006, 4:52pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174/2 "2006-07-04T16:52:38Z")
[An answer to your last question.](http://www.straightdope.com/classics/a3_144.html) ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 4, 2006, 4:52pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174/3 "2006-07-04T16:52:57Z")
No, this is the last question: Why should _we_ do your homework for _you_? ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [July 4, 2006, 4:58pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174/4 "2006-07-04T16:58:33Z")
[QUOTE=smile4me2222] 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? [/QUOTE] I think this is meant to be a comment on Cecil’s [column regarding your last question](http://www.straightdope.com/classics/a3_144.html), 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_. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 4, 2006, 7:32pm UTC](https://boards.straightdope.com/t/which-has-greater-mass/363174/5 "2006-07-04T19:32:38Z")
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.