In my chemistry textbook, there is a table of the molar volumes for different gases. Most of them are below 22.42 l/mol, the ideal gas molar volume, but I noticed that hydrogen and helium do not. Hydrogen is 22.433 l/mol and helium is 22.434 l/mol. Why is this? What makes these particles, with mass, have higher molar volumes than the basically mass-less particles of an ideal gas? And why does the molar volume increase when going from Hydrogen to Helium?

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    $\begingroup$ Mass is hardly relevant here. The deviation from ideal molar volume is an interplay of two factors, both of which are absent in ideal and present in real gases: (1) volume of molecules themselves and (2) attractive interactions between molecules. $\endgroup$ – Ivan Neretin Aug 16 '16 at 20:24
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    $\begingroup$ And of those two factors, the nonzero volume may outweigh the attractive force in the case of hydrogen and (to a greater extent) helium, giving a positive deviation from the ideal gas volume--whereas attractive forces dominate in the other gases, causing a negative deviation from ideal volume. $\endgroup$ – iad22agp Aug 16 '16 at 21:06
  • $\begingroup$ Are these tiny differences between hydrogen and helium really significant? $\endgroup$ – Chet Miller Aug 17 '16 at 13:20
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    $\begingroup$ @ChesterMiller The major differences that come about from the tiny differences in IMF's are in the boiling points (H2: -252.9 degC, He: -268.9) [values from wikipedia] $\endgroup$ – Dan Burden Aug 18 '16 at 20:17

As hydrogen and helium's molecular weight are too less, the intermolecular attractions are also too less. So the a/V^2 is negligible. So the v.d.o equation becomes P (V - nb) =RT . By which, it can be said as the pressure of these elements are too less and from Boyle's law , the volume is higher.


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