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Why does one mole of all gases occupy 22.4 L at standard temperature and pressure? According to my thinking, it may be due to equal diffusion of all particles when at a particular temperature and pressure.

So going by this, one mole of all gases should occupy the same $x~\mathrm{L}$ at some other temperature-pressure conditions. Is this true?

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Why do all gases occupy 22.4 L [per mol] at STP?

The question is based on a false premise. Only ideal gases are guaranteed to occupy 22.4 L/mol at STP. There are many gases that are not ideal.

So going by this, all gases should occupy same $x$ L at some other temperature-pressure conditions. Is this true?

No, again this would be true only for ideal gases, not all gases.

At STP, many commonly encountered gases on Earth behave nearly ideally, but certainly not all. For example, vapor of acetic acid and other simple carboxylic acids show strong deviations from ideality.

Wikipedia is a good place to read about the assumptions of the ideal gas law and the kinetic theory of gases. The reason that ideal gases share a molar volume of 22.4 L at STP is not related to diffusion, but rather to the assumption of: (a) non-interacting gas particles that are (b) effectively zero-sized relative to the mean distance between them.

You can read about equations of state for non-ideal gases on Wikipedia as well. Popular equations to describe non-ideal gases include the van der Waals equation, the Redlich-Kwong equation, and a whole slew of others listed at Wikipedia's real gas page.

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    $\begingroup$ Thank you that was very helpful! Actually i was taught .. quoting exact words " all gases occupy 22.4 L at STP" $\endgroup$ – Max Payne Apr 23 '15 at 14:04
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    $\begingroup$ I love this answer! Exactly helpful to a learner! +1 since I can't upvote more than once. $\endgroup$ – M.A.R. Apr 23 '15 at 16:46
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    $\begingroup$ Although, technically no gas is ideal, which is why we can be so certain of their properties. $\endgroup$ – PyRulez Apr 23 '15 at 23:08
  • $\begingroup$ I agree with PyRulez. No gas is ideal. It would be good to change that so that it isn't misleading. $\endgroup$ – Clangorous Chimera Dec 31 '16 at 1:34
  • $\begingroup$ This "no gas is ideal" idea needs empirical support. What's the deviation from ideality for say helium 4 at .00001 PA pressure and a temperature of 1000 K $\endgroup$ – Curt F. Dec 31 '16 at 1:38
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IUPAC has changed the definition of STP from [273 K, 1 atm (101.3 kPa)] to [273 K, 1 bar (100.0 kPa)]. Hence their molar volume at STP has changed from 22.4 L/mol to 22.7 L/mol.

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    $\begingroup$ do you have a reference you can include for the change of STP? $\endgroup$ – user15489 Apr 24 '15 at 23:57
  • $\begingroup$ Reference: Ideal Gas wikipedia article mentioned above, and follow the references therein. $\endgroup$ – Trent Pattee Apr 25 '15 at 14:54
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    $\begingroup$ I didn't realize that, thanks for the info. Also, why in the world would IUPAC do that? It's hard to think of a reason other than the self-perpetualization. $\endgroup$ – Curt F. Apr 26 '15 at 4:25

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