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Name: Paul E. P.
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001


Question:
Can you suggest a simple means of massing helium gas? If so, I would appreciate your help in this matter.



Replies:
The accurate mass of atoms and molecules is most commonly done with modern instrumentation by high resolution mass spectrometry, which actually measures the ratio of the mass to the charge of the ionized atom or molecule. The principles of mass spectrometry can be found on the web site:

http://masspec.scripps.edu/FINALCOPY.htm

Vince Calder


I am not sure why one would try to weigh helium gas rather than measuring its volume at a known pressure and temperature and using the ideal gas law: PV=nRT, where P = pressure in atm, V = volume in liters, n = mass/molecular weight, R = the universal gas constant

(0.0825 liter*atm/mol*K) and T = temperature in kelvins.

Not withstanding, if you weigh the helium in a rigid container, like a glass or metal, the buoyancy of helium (or anything else) in the rigid container is determined by the volume of displaced atmosphere. This is the same so it tends to cancel when the rigid container is tared. You can find the buoyancy correction discussed in any undergrad text on analytical chemistry.

Vince Calder



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