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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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