Density of gas

[chiddler]

In elemental gases (F, N, O) why is the highest density gas the one with the lowest mass, namely nitrogen? I'd expect intermolecular forces to increase with larger atomic size specifically london forces making F the highest density.

Also what is the difference between van der waals and london forces?

---

Members don't see this ad.

 

[MedPR]

In elemental gases (F, N, O) why is the highest density gas the one with the lowest mass, namely nitrogen? I'd expect intermolecular forces to increase with larger atomic size specifically london forces making F the highest density.

Also what is the difference between van der waals and london forces?

Fluorine does have the highest density.

Why would intermolecular forces affect density? Density is mass/volume. Nitrogen has the lowest density because as you move from right to left on the periodic table, atomic radius (volume) decreases. However, since Nitrogen, Oxygen, and Fluorine are all limited to the 2p shell, they have very similar atomic radii (volume). As you move from left to right across the periodic table, you gain mass due to the increasing number of protons. Nitrogen has 7, Oxygen has 8, Fluorine has 9, etc. An increase in mass with a negligible increase in volume = increased density.

Also, from wiki, London forces are a type of van der Waal forces. Specifically those between induced dipoles.

http://en.wikipedia.org/wiki/Van_der_Waals_force

 

[Ssina]

how about if we are talking about N2 (4.614 kg/m3) vs. O2 (4.475 kg/m3) vs. F2 (1.7 kg/m3)?!

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=5
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=48
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=84

 

[chiddler]

Cool, I misread the question lol. It's which has the least density.

Intermolecular force would increase density because a gas with high intermolecular forces would be more compact. So more mass in a given volume compared with a gas with low intermolecular forces = less mass in same volume.

However, I think you're more right. It's pretty much negligible and the more important consideration is that there is less mass per unit volume for nitrogen.

Thanks again for the help.

 

[MedPR]

how about if we are talking about N2 (4.614 kg/m3) vs. O2 (4.475 kg/m3) vs. F2 (1.7 kg/m3)?!

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=5
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=48
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=84

You're comparing data measured differently.

From those links...

For N2:
Gas density (1.013 bar at boiling point) : 4.614 kg/m3
Gas density (1.013 bar and 15 °C (59 °F)) : 1.185 kg/m3


For F2


Gas density (1.013 bar and 0 °C (32 °F)) : 1.7 kg/m3
Gas density (1.013 bar and 15 °C (59 °F)) : 1.59 kg/m3



1.59 kg/m3 (F2) > 1.85kg/m3 (N2) 🙂

 

[MT Headed]

how about if we are talking about N2 (4.614 kg/m3) vs. O2 (4.475 kg/m3) vs. F2 (1.7 kg/m3)?!

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=5
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=48
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=84

Your comparisons are invalid because they are not under the same conditions. Specifically your measurements are at different temperatures. O2 was at its boiling point, which is ver cold and dense, while the F2 measurement was at zero C.

All gases will be 22.4 liters per mole under standard temp and pressure. So it is really a matter of which gas has fewer grams per mole since for a given volume they will all have the same number of moles..

Interesting trivia point: air is mostly N2, so air is more or less 28 g/mol. Slightly more because of the presence of O2 which is even heavier. Anyway, what happens if the air is moist? Does the density go up or down? Most people think humid air is 'heavy' and therefore denser. But water is only 18 g/mol, so the presence of water makes air less dense! This tripped me up on a practice passage once.

 

[MedPR]

Your comparisons are invalid because they are not under the same conditions. Specifically your measurements are at different temperatures. O2 was at its boiling point, which is ver cold and dense, while the F2 measurement was at zero C.

All gases will be 22.4 liters per mole under standard temp and pressure. So it is really a matter of which gas has fewer grams per mole since for a given volume they will all have the same number of moles..

Interesting trivia point: air is mostly N2, so air is more or less 28 g/mol. Slightly more because of the presence of O2 which is even heavier. Anyway, what happens if the air is moist? Does the density go up or down? Most people think humid air is 'heavy' and therefore denser. But water is only 18 g/mol, so the presence of water makes air less dense! This tripped me up on a practice passage once.

I probably would've gotten tricked on that too. Isn't moist air "heavier"? Thus greater mass in the same volume? I don't understand how/why it would be less dense 🙁 😱

Edit: Nevermind, 18g/mol < 28g/mol. Why then is it harder to hit homeruns in humid air than in dry air?

 

[anthony2009]

Thanks for informative posting.Metaburn

 

[Ssina]

Thanks for informative posting.

👍
yeah totally missed that, almost freaked out there for a sec!
Thanks MedPR and MT Headed!