# Why does the pressure-volume-constant of Helium increase?

## Summary:

Why does the pressure-volume-constant of Helium increase with increases of external pressure; when the pressure-volume-constant decreases with increases of pressure for other real gasses?
Im doing a lab on an online software called beyond labs. On this software I am able to test gas laws by adding ideal and real gasses to a balloon in a pressure chamber.

When I am conducting the test I have a consistent temperature of 298K and .300 moles across all the tests; the only variables being manipulated is the pressure as the independent variable, and volume as the dependant variable.

For all my tests on real gasses as the pressure increases the volume decreases; Avargadro's Law: V=kn.
Subsequently, the pressure-volume-constant decreases because real gasses do not act idealistically. however with Helium gas as I increase the pressure, the pressure-volume- constant is on an increasing trend.

Why is this the case?

hutchphd
It shows up in the Van der Waals modification to the ideal gas law. The two positive coefficients a and b

[P + a(n/V)2] (V/n - b) = RT

take into account interparticle long range attraction and short term repulsion (size effects) respectively. It is not hard to see that they work in different directions on the PV product. For Helium the a is very small,. I think that explains it.
Physically this is because the long distance attraction is electric dipole-dipole and the noble gases are very small and spherical.

• TeethWhitener and Tom.G
Borek
Mentor
Can't say I ever heard about the "pressure-volume-constant".

Do you mean value of the pressure*volume product? It is constant only for ideal gases, or real gases in the range where they can be treated as ideal.

• jim mcnamara
TeethWhitener
$$Z=\frac{pV_m}{RT}$$
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