[wein3967]

My basic problem is a vessel containing hydrogen gas (H2) at 4,500 psig near its autoignition point (500 C). I'd like to calculate the mass of hydrogen present. Due to the high pressure, I am considering using van der Waals (VDW) over the ideal gas law (other EOS suggestions are welcomed). When I perform the VDW calc and get the number of moles, is that number for monoatomic or diatomic hydrogen? I'd assume that specification is determined by the "a" and "b" values, but the references don't clarify which one. If I had to guess, I'd say it is the moles of diatomic hydrogen. Thanks.

[pgk]

Van der Waals constants depend on the critical temperature and the critical pressure of the gas.
Van der Waals constants for Hydrogen, are:     
a = 0.2452     
b = 0.0265
Reid, R.C, Prausnitz, J. M., and Poling, B.E., The Properties of Gases and Liquids, Fourth Edition, McGraw-Hill, New York, 1987
Question: The molecule of hydrogen in gaseous state, is it monoatomic or diatomic?

[wein3967]

Thanks pgk. Getting the VDW constants is not an issue. They are readily available from lots of references in several units. Since hydrogen is a diatomic, in its gaseous form, it would be H2. This is why I'd be inclined to think that solving VDW for "n" would results in moles of H2 not H. I just want to verify that.

[pgk]

Gas laws and their virial expansion refer to molecules.
Argon is monoatomic, oxygen is diatomic, ozon is triatomic, gaseous sulfur is polyatomic and so on.