• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Does one molecule of H2O weigh the same as 2 Hydrogen and 1 Oxygen?

P

Penth

Senior member
I was having this argument from another thread and one of their arguments that you could gain more weight than something weighs is that an H20 molucle w weighs more than two hydrogen and one oxygen molecule. They were saying hydrogen has no weight because it's lighter than air. I said I didn't know enough about it to say it weighs more or less but thinking about it now I think it weighs the same and probably the density just changes.

What are your thoughts?

Edit: Updated link!
 
I dunno. Could it be a difference between saying mass and weight? Isn't the whole idea of the "mole" to characterize the mass of different atoms?

I havent had chemistry since highschool, so i'm pretty clueless...
 
An atom of H2O actually has slightly less mass than two hydrogen atoms and one oxygen atom due to relativity. However, this difference is a fly on a cruise ship. Because of buoyancy, the gaseous forms rise, just as steam does. So they do "weigh" less. But given the same number of atoms, the amounts will be approximately the same mass.
 
Bignateyk hit the nail on the head. Mass and weight are two different things.

And yes, the mass will be the same, otherwise the whole concept of stoichiometry would be thrown out the window.
 
Originally posted by: Penth
I was having this argument from another thread and one of their arguments that you could gain more weight than something weighs is that an H20 molucle w weighs more than two hydrogen and one oxygen molecule. They were saying hydrogen has no weight because it's lighter than air. I said I didn't know enough about it to say it weighs more or less but thinking about it now I think it weighs the same and probably the density just changes.

What are your thoughts?
Click to expand...

Your link is bad and I think I should thank you for not allowing me to witness these posts. The H20 molecule has about the same mass as two H and one O atoms because they bond via covalence bonds. However, it's a bit of a moot point because oxygen and hydrogen would naturally exist as diatomic gases while H2O would be a liquid. The densities and the configuration of states of these two prevent a direct comparison like they are saying.
 
Actually, because an H20 molecule has less energy than 2 hydrogen atoms and an oxygen atom, H2O will have very slightly LESS mass than the three atoms floating around independantly.


Just saying.
 
Originally posted by: LordMorpheus
Actually, because an H20 molecule has less energy than 2 hydrogen atoms and an oxygen atom, H2O will have very slightly LESS mass than the three atoms floating around independantly.


Just saying.
Click to expand...

As noted above though, the difference is so small as to be practically negligible.
 
I agree with the less, I think the most important point though is it would not be *more* in any circumstance, right?
 
No, a water molecule actually weighs a very very small amount less than its individual components.

Also, the chemical reaction that occurs on water when we metabolize it does not result in Hydrogen gas being formed. (else of course our exhaled gases would always be flammable) It is usually consumed by the body and used to create hydrogen ions for Acids and other compounds.

<= Chemistry Major
 
Originally posted by: Acanthus
No, a water molecule actually weighs a very very small amount less than its individual components.

Also, the chemical reaction that occurs on water when we metabolize it does not result in Hydrogen gas being formed. (else of course our exhaled gases would always be flammable) It is usually consumed by the body and used to create hydrogen ions for Acids and other compounds.

<= Chemistry Major
Click to expand...

I think you forgot a few "very"s. 😛
 
Originally posted by: Lorax
"hydrogen has no weight because it's lighter than air"

wtf?
Click to expand...

Talking about gaseous form, I guess you could say it has no weight on Earth (but still mass of course). But would definitely have weight on Jupiter, Saturn, or the Sun.

Still a horrible way to word it, and I doubt that's what they meant anyway.
 

"hydrogen has no weight because it's lighter than air"


Lighter means it weighs less, not that it has no weight. Does a dead whale have no weight because it floats in water?

Also, for the "negligible" amount of mass that's lost: it's pretty simple to calculate:
E=mc² Look up the energy for the reaction of hydrogen with water.
 
(delta)Heat of formation for water vapor is -241.8 kJ/mol.

Dividing that by (3.00e8 m/s)^2 gives 2.69e-12 kg/mol or 2.689e-15g/mol.

2 moles of H and one mole of O add up to 18.02g/mol, so the percentage of matter converted would be 1.49e-14 %. Completely negligible.

Edit: I could be completely wrong in my method, just my first thought as to how to find the value.
 
Originally posted by: DrPizza

"hydrogen has no weight because it's lighter than air"


Lighter means it weighs less, not that it has no weight. Does a dead whale have no weight because it floats in water?

Also, for the "negligible" amount of mass that's lost: it's pretty simple to calculate:
E=mc² Look up the energy for the reaction of hydrogen with water.
Click to expand...

Hence my comment of "fly on a cruise ship". Does the fly increase the mass? Yes. Is it measurable? Conceivably. Does anyone care? No.
 
Originally posted by: MrPickins
Originally posted by: LordMorpheus
Actually, because an H20 molecule has less energy than 2 hydrogen atoms and an oxygen atom, H2O will have very slightly LESS mass than the three atoms floating around independantly.

Just saying.
Click to expand...

As noted above though, the difference is so small as to be practically negligible.
Click to expand...

But since we're arguing on a theoretical standpoint, we should include all facts.
 
Originally posted by: TuxDave
Originally posted by: MrPickins
Originally posted by: LordMorpheus
Actually, because an H20 molecule has less energy than 2 hydrogen atoms and an oxygen atom, H2O will have very slightly LESS mass than the three atoms floating around independantly.

Just saying.
Click to expand...

As noted above though, the difference is so small as to be practically negligible.
Click to expand...

But since we're arguing on a theoretical standpoint, we should include all facts.
Click to expand...

This thread may be theoretical, but the thread the OP referenced was purely practical.
 
Originally posted by: Demon-Xanth
An atom of H2O actually has slightly less mass than two hydrogen atoms and one oxygen atom due to relativity. However, this difference is a fly on a cruise ship. Because of buoyancy, the gaseous forms rise, just as steam does. So they do "weigh" less. But given the same number of atoms, the amounts will be approximately the same mass.
Click to expand...

Heh, I was about to point out the tiny, tiny, tiny mass difference between H2O and 2H + O, but you beat me to it.

Bouyancy != weight, though. Many weight-measuring devices rely on non-bouyant items for accuracy, but a truck full of hydrogen still has to hit the brakes harder than a truck full of vacuum (yes, I know they don't ship vacuum from place to place😉).

Originally posted by: Acanthus
No, a water molecule actually weighs a very very small amount less than its individual components.

Also, the chemical reaction that occurs on water when we metabolize it does not result in Hydrogen gas being formed. (else of course our exhaled gases would always be flammable) It is usually consumed by the body and used to create hydrogen ions for Acids and other compounds.

<= Chemistry Major
Click to expand...

Our net metabolism actually produces water. Not to say that we don't split it in a number of other reactions, but the general form is C12H22O11+12 O2 -> 11 H2O + 12 CO2.

Of course you're right about H2 not floating around as a free gas.
 
I don't remember which way it goes (or if it has been answered already) but when you combine and split atoms from molocues there is mass lost or gained due to the reaction. So the 2 are not equal in mass. It's very minor but still there.

As for "They were saying hydrogen has no weight because it's lighter than air." that definatly qualifies for the stupidest comment for the day.
 
Can't you not have just one oxygen atom by itself since it's diatomic?
So shouldn't it be 2H2+O2=2H2O?
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top