weight of hydrogen ?

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
Just wondering as one does,

about pressures / weights

So thought process went like this ,,,,

If I have a helium generator of some sort,
and the gas is at the same temperature as the ambient
and a tube above the generator, say 100 m long tube, 1 m diameter
and at the top of the tube I had a small hole, and the bottom of the tube is open to the air,
may be put the bottom of the tube under water, to seal the gas being made,

Assuming the generator makes enough He to fill the tube,

The He rises into and fills the tube,

So what is the mass and weight of the He ?

would the pressure at the top be higher than at the bottom as the gas is rising ?
what would the pressure be ? I guess assuming same atmosphere pressure at top and bottom of the tube.


No its not home work,
many many decades since I went to school,
just a random thought
 

Papabravo

Joined Feb 24, 2006
16,461
Consult a periodic table of the elements. In that table there will be a number for each element, called the atomic weight. The units of this number are "grams per mole". One mole of any element in the periodic table is 6.02e23 atoms of that element.

If you know the pressure, volume, and temperature of a gas you can use the ideal gas law to compute the number of moles, n, of gas that you have. Multiply by atomic weight and you get mass in grams.

\[ PV\;=\;nRT \]

Watch the units very carefully, especially the units of the Rydberg constant, otherwise your results will be nonsense.
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
Sorry no trick question,
I just work up this morning wondering ,, Obviously had a bad night , to much lock down,

I assume the generator would make enough to fill the tube,

I know he is a slippery as Heck,
back in the old days, I've worked with it when its liquid, and it seems to climb out the helio stat thing or what ever it was called.,

It was just a thought,
wondering what the pressures would be ,
 

bogosort

Joined Sep 24, 2011
678
If I have a helium generator of some sort,
and the gas is at the same temperature as the ambient
and a tube above the generator, say 100 m long tube, 1 m diameter
and at the top of the tube I had a small hole, and the bottom of the tube is open to the air,
may be put the bottom of the tube under water, to seal the gas being made,

Assuming the generator makes enough He to fill the tube,

The He rises into and fills the tube,

So what is the mass and weight of the He ?

would the pressure at the top be higher than at the bottom as the gas is rising ?
what would the pressure be ? I guess assuming same atmosphere pressure at top and bottom of the tube.
I'm not clear on your experimental setup (the tube has a hole at the top?), but I think I get the gist of what you're wondering about. To simplify the calculations, we can say that we have a closed container with a mole of pure helium. What is the mass of the He in the container? Since a a mole of hydrogen is one gram of hydrogen, and helium has four times the mass of hydrogen, the mass of the helium gas is 4 grams.

Since you know the mass of the gas, you can simply derive the weight of the gas from the nominal value of gravitational acceleration. If, however, you'd rather experimentally determine the weight of the gas, that is much trickier. Even if you could completely evacuate the container (which you can't), it'd be difficult to accurately measure the difference between the weight of the empty container and the weight of the He-filled container. And even if you found a scale capable of resolving that tiny difference, you'd have to account for the buoyancy effect, a tricky experiment in its own right. But weight isn't a very meaningful metric -- it's relative to whatever scale was used -- so I wouldn't bother. :)

As for the pressure inside the container, you could treat the gas as ideal and calculate it from the known volume and temperature. In any case, I would suppose that the pressure is equivalent on all sides of the container. The gas is pure He, so there is no density gradient within the container.
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
Thank you,.

I have no access to He or H, it was a pure thought that I woke with
As for pressure,

if the ambient was air, I'd assume the gas was primevally N2, and use that atomic weight,
If the gas in the tube was He,
which is lighter than air,
I'd say the pressure at the top of the tube would be more than at the bottom
( sort of intuitive , as if it was a tube of water, with a small hole at the bottom, and tap at the top , the bottom would be higher pressure )

If I have a few hours free, I'll have a go at calculating this.
any one want to check my numbers
 

jpanhalt

Joined Jan 18, 2008
11,088
The labware we used to weigh gases in analytical chemistry was like a small round or flat bottom flask, but instead of a stopper, it had a spiraled capillary. I couldn't find a picture. Maybe they aren't used anymore.

The volume of the vessel was accurately known (or could be measured). The open capillary would allow mass flow due to differences in pressure, but once the pressures equalized, loss of gas was limited to diffusion and was considered to be slow for most gases.

Thus, you needed an accurate room pressure and temperature. Then one filled the container, let it equilibrate for pressure and temperature (often done by waiting for a stable weight), weighed it, and corrected for buoyancy in air.

In your description, a 100 m tube could show significant differences in pressure at either end, particularly if held vertical by some mechanism.
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
Thank you
I am wondering what the pressure difference is in this thought experiment,
never intending to do it practicaly,
 

bogosort

Joined Sep 24, 2011
678
If the gas in the tube was He,
which is lighter than air,
I'd say the pressure at the top of the tube would be more than at the bottom
( sort of intuitive , as if it was a tube of water, with a small hole at the bottom, and tap at the top , the bottom would be higher pressure )
There is no air in the tube, right? So why would the helium rise within the container? If the tube is filled with pure helium, then we can model the system as a bunch of energetic helium atoms bouncing around, off each other and off the walls of the container. Within the container there should be no net pressure differences. The outside of the container is a different story and depends on how much air the container displaces.
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
There is no air in the tube, right? So why would the helium rise within the container? If the tube is filled with pure helium, then we can model the system as a bunch of energetic helium atoms bouncing around, off each other and off the walls of the container. Within the container there should be no net pressure differences. The outside of the container is a different story and depends on how much air the container displaces.

Why should the He rise you ask ?
well I was assuming as top and bottom are open to ambient, then the He would rise .
 

joeyd999

Joined Jun 6, 2011
4,477
may be put the bottom of the tube under water...
To make it even more confusing, water has a temperature-dependent vapor pressure, and will contribute as a portion of the total pressure/mass of gas in the tube. This is normally non-negligible in these kinds of experiments, especially considering a > 4:1 molecular mass/atomic mass ratio.
 

K OBrien

Joined Nov 28, 2020
14
The Volume is PI (R^2)*L or 3.14*1*100= 314 cubic meters. You didn't say anything about how fast the gas is generated or how fast its fills or how fast it escapes. As the second poster said you now need to know the pressure assuming its open at sea level then that would be one atmosphere or about 101kPA. The change in atmospheric pressure would be small between the top and bottom of the 100 meter tube. It would be much like climbing a 100 meter hill i don't think your ears would notice it and the normal weather probably has bigger changes since a normal barometric pressure is about 30 inches of mercury and you might hear a weather forecast of about 29.5. You would still need to know the absolute temp in degrees Kelvin or Celsius plus 273. You could call it 300 degrees kelvin (87 degrees F) now you have P*V=N*R*t or PV/Rt=N with R as the Ideal Gas Constant. R is 8310 in m^3*kPa/K*mol (101*314)/(8310*300)= .0127 moles of gas helium weighs about 4 grams per mole making it .0508 grams of gas.
 

K OBrien

Joined Nov 28, 2020
14
To make it even more confusing, water has a temperature-dependent vapor pressure, and will contribute as a portion of the total pressure/mass of gas in the tube. This is normally non-negligible in these kinds of experiments, especially considering a > 4:1 molecular mass/atomic mass ratio.
Good Point!!! Plus puting it underwater with a very small hole would allow pressure to build.
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
Oh, that's entirely different. If the container is not sealed the He will simply escape and you will have a container full of air.
Sorry @bogosort

I did say at the beginning of this thought experiment , there is hole at the top, at the bottom is a generator capable of filling the tube,
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
To make it even more confusing, water has a temperature-dependent vapor pressure, and will contribute as a portion of the total pressure/mass of gas in the tube. This is normally non-negligible in these kinds of experiments, especially considering a > 4:1 molecular mass/atomic mass ratio.
@joeyd999

yeh, its was just a thought about under water generator

I don't want this to get crazy...

take it as it is,

its a thought I woke up with , and wondered what the weight of a tube of He open at both ends would be .

I gave it some parameters so people would not go to the extreme,
 

Thread Starter

andrewmm

Joined Feb 25, 2011
1,612
@KOBrien

Thank you,
about 0.05g is a number I like the look of.


I'm also thinking this must be hooked into He balloons,

I have looked up the air around is about 29g/Mol
I've just been reading that 1 Mol of any gas is 24 000 cm^3
so a balloon of 1 Mol of He in the atmosphere would displace 1 Mol of air ,
which would give about 29 g of lift. ( excluding the weight of the balloon etc etc )

You know , 50 years after leaving school I think I'm getting the idea of Mol.

Thank you guys.

Wonder what the pressure would be in the top of the tube !
 

K OBrien

Joined Nov 28, 2020
14
Maybe you could build a really lightweight balloon and dangle an old cell phone from it such that the camera points down then call the phone and watch the scenery as the ballon travels.
 
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