# The Naked Scientists Forum

### Author Topic: How Do You Weigh A Gas Molecule ?  (Read 6699 times)

#### neilep

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##### How Do You Weigh A Gas Molecule ?
« on: 01/05/2008 13:23:59 »
Hi ya,

How are ewe today ?...My ear is still a bit iffy !....thanks for asking !

See The Hydrogen Gas Molecule ?

There it is .....all being a busy hydrogen molecule !

But how do we weigh gas ?.....it can't be easy to get a gas molecule to stay still !...I tried bottom-burping on some weighing scales but it didn't work.. (ewe try it !)

So how do we tell the weight of gas ?

Thanks

Neil
xxxxxxx

#### another_someone

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##### How Do You Weigh A Gas Molecule ?
« Reply #1 on: 01/05/2008 15:16:44 »
There is no such thing as a gas molecule.  A molecule is a molecule is a molecule.  Gasses, solids, and liquids are collections of molecules, but a single molecule is none of these things.

There are two ways I can think of weighing molecules:

1) weigh lots of them together, and then count how many there are.  In terms of gases, this is made easier because we know that a given volume of gas always contains the same number of molecules, so if you know how many molecules exist in a cubic metre of gas (which we happen to know to be around (2.686 7774 ± 0.000 0047)×1025 m−3), you just weight one cubic metre of the gas, and divide it by this number.

Alternatively, you can simply measure the weight of one molecule in a mass spectrometer.

#### Bored chemist

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##### How Do You Weigh A Gas Molecule ?
« Reply #2 on: 01/05/2008 17:53:59 »
"Alternatively, you can simply measure the weight of one molecule in a mass spectrometer. "
Actually, since mass spectrometers would generally work perfectly well in zero gravity I don't think they measure weight. They measure mass- the clue's in the name.

#### lyner

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##### How Do You Weigh A Gas Molecule ?
« Reply #3 on: 13/05/2008 11:09:50 »
Actually, gases are 'heavier' than you might think. A 1 litre flask 'weighs' measurably different when it is full of air from its 'weight' with most of the air pumped out. You can demo it to School kids with a bog standard School chemical balance (1.3g difference - that's 0.013N, weight, actually - assuming g=10N/kg).

#### The Naked Scientists Forum

##### How Do You Weigh A Gas Molecule ?
« Reply #3 on: 13/05/2008 11:09:50 »