Different nitrogen vs "air" question

[George Bynum]

A buddy told me he'd been told that an average car tire/wheel filled with nitrogen weighed "several pounds" less than one filled with plain old air. I thought a while, did some swagging, and said lighter, but I didn't even think several ounces. Capri, where are you? Are my assumptions in the ballpark? I'll do a mixture of imperial and metric units and convert approximately convert as I feel like it. I'm thinking of a "normal" tire, perhaps in the range of 255-55/17, but kinda feel the best, standard rating correlation with volume might be with LoadRating, so maybe in the 90-100 range?

I "assume" ...
a torus 7" diameter (~2 gallon/ft)
average circumference ~4.5 ft
volume thus ~9 gallons ... let's assume 44.8 liters for later
1 mole is 22.4 liters at STP
1 mole of N2 weighs 28 grams
1 mole of "air" weighs 29 grams
I don't see that absolute units are important ... to go from 1 bar absolute to 3 bars absolute (about 30 psig) requires 2 volumes added to "empty" tire, or ~4 moles.
External buoyancy won't change, so ignore it.

SO, wild wild wild SWAG ... N2 filled auto tire weighs perhaps between 0.1 and 0.4 ounces less than 1 filled with air. I might say it'd average 1/4 ounce lighter across sizes and inflation pressures ...

Experts, imperts, geniuses, idiots, in between ... chime in. All I'm sure of is that I have too much time on my hands.

 

[Astro14]

I'm impressed, your high school chemistry looks good to me...

Once again, ridiculous myths on N2 inflation abound...glad to see you apply some logic to the question...

 

[AVB]

So what was your reply to your friend? Did you tell him that was only if he used "several pounds" less pressure?

 

[stockrex]

The main selling point for nitrogenis that it is dry and does not ruined the sensors.
The weight reduction will be in the ratio of air/nitrogen weight ratio.

 

[PandaBear]

I don't think even pure nitrogen weight several pound compare to vacuum.

 

[dailydriver]

I would not even bother arguing with him if he can seriously make a statement/claim like that one.

 

[Jeff_in_VABch]

I think you have mis-applied the ideal gas law and you have the wrong units for your atomic weights.

The atomic weight of N is 14 and one mole = 14 grams. The atomic weight of O is 16 and one mole = 16 grams.

Both gases are diatomic and exist as N2 and O2 in this application. However the weight per mole is still 14 & 16 grams respectively. A mole of "air" (at 75%N/25%O) would be 15.5 grams or 10% heavier than the nitrogen alone. Of course I dont think the math is this simple as a mole is a parametric rather than a metric.

Gotta go for now, but I was thinking the difference in weight is much less than 1/4 ounce.

 

[AVB]

I just thought of something, if air leaks out through the rubber like nitrogen sellers advertise wouldn't the tire eventually purge out the oxygen and other smaller trace gasses and leave the nitrogen behind?

 

[Astro14]

Originally Posted By: AVB
I just thought of something, if air leaks out through the rubber like nitrogen sellers advertise wouldn't the tire eventually purge out the oxygen and other smaller trace gasses and leave the nitrogen behind?


Partial pressure - means that the pressure of O2 on the inside and outside eventually equalize...so, no, it will never get to zero...and if the O2 starts at zero inside...it will leak in to the tire...but we've been over this so many times, I am tired of typing...

 

[stockrex]

what is heavier 1 pound of nitrogen or 1 pound of air

 

[Quattro Pete]

Originally Posted By: stockrex
what is heavier 1 pound of nitrogen or 1 pound of air

Jokingly or not, that isn't the right question in this scenario. The question would be: what is heavier: one cubic foot of feathers or one cubic foot of concrete?

Except that the difference between oxygen and nitrogen won't be nearly as pronounced.

 

[Carbon]

Originally Posted By: Jeff_in_VABch
The atomic weight of N is 14 and one mole = 14 grams. The atomic weight of O is 16 and one mole = 16 grams.


http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#mole
"One mole of a molecular compound contains Avogadro's number* molecules and has a mass equal to the substance's molecular weight, in grams."

So a mole of N2 would be 28 grams.

 

[Shannow]

 

[JHZR2]

I think you did good in your analysis.

 

[Jeff_in_VABch]

Originally Posted By: Carbon
Originally Posted By: Jeff_in_VABch
The atomic weight of N is 14 and one mole = 14 grams. The atomic weight of O is 16 and one mole = 16 grams.


http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#mole
"One mole of a molecular compound contains Avogadro's number* molecules and has a mass equal to the substance's molecular weight, in grams."

So a mole of N2 would be 28 grams.


You are right but technically wrong. A mole is a parametric, not a metric. You cant add them up.

 

[Shannow]

A mole of a molecule is avagadros number, regardless of how many atoms make up the molecule.

A mole of CO2 is 44 grammes, and takes up 22.4L, while a mole of helium is 4 grammes and takes up the same space. (at STP)