How closely, if at all, does helium in the supercritical phase obey PV=nRT?
I think you have to use the VanDer Waal Equation for real gasses here which is
Here the terms a and b are dependent on the critical pressure and temperatures of the gas in question.
a=27R^2Tc^2/64Pc and b=RTc/8Pc
where Tc is the critical temperature and Pc is the critical pressure.
To determine how ideal helium will be have you can also find the compressibility factor z. For an ideal gas z always equals 1.
for a real gas
From that you can see how much helium will deviate from an indeal gas behavior.
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