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From outside the quantum stuff I'm tiredelessly learning, I've been reminded about Avagadro's Law
— i.e., the fact that any molecular gas has the same count of molecules in the same volume and temperature.

That was a bit surprising because I think about solids more often; with quantum properties of their nuclei propagating up to the macroscopic level (namely, differing them into conductors, semiconductors, magnets, etc.).

But then I thought that the reason why gases are different is because in normal conditions (i.e., if gas is not ionized, or whatever), their molecules must be neutral; therefore, indeed, any gas is just a collection of EM-neutral floating balls: it doesn't matter how heavy they are.

So, the sole-reason for Avagadro's Law is EM-neutrality, is that correct?

EDIT: so, the question was about real gases; not idealized models.

Victor Novak
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This is true only for an ideal gas i.e. a gas that obeys the equation of state:

$$ PV = nRT $$

A rearrangement gives us the molar density:

$$ \frac{n}{V} = \frac{P}{RT} $$

and for constant $P$ and $T$ the molar density is a constant, hence Avagadro's law.

It is certainly true that the gas molecules have to be neutral to be ideal because there cannot be any intermolecular forces in an ideal gas. However there are other restrictions as well e.g. the molecules must have a zero volume.

Showing that an ideal gas obeys the equation above is simple enough if you don't mind a hand waving argument. See for example my answer to Why/How is $PV=k$ true in an ideal gas?

John Rennie
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  • That's fine with me! But how close to 'ideal' are real gases, in normal (i.e., 'not ionized, etc.') conditions? Is that approximately o.k. that real molecules do have volume besides being neutral? Or it's more complicated?.. – Victor Novak Nov 16 '22 at 08:59
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    @VictorNovak Under everyday conditions gases such as the air are very close to ideal. This because the spacing between molecules is large compared to the range of the forces acting between molecules and also large compared to the radii of the molecules. That is, the molecules spend enough time far apart from each other that they are not significantly affected by the other molecules. – John Rennie Nov 16 '22 at 09:06
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    @VictorNovak It depends on the gas and the thermal properties of the system you are looking at. The ideal gas is a good description if, roughly speaking, the "temperature"-energy-scale is high with respect to the "interaction strength"-energy-scale of the gas molecules. Furthermore the effects of the finite volume of the particle need to be negligible with respect to the overall volume within the gas is confined. Last but not least there must be a sufficient amount of gas to get a sensible description using thermodynamics. – AlmostClueless Nov 16 '22 at 09:09