One of my teacher said that the one with more molecular mass will deviate more from behaving as an ideal gas and he showed a graph given below.

enter image description here

Is his statement is correct? If no, then what's the reason?

In th graph $\ce{CO2}$ have higher molecular mass than other gases and its deviation is more than others, so from his point of view his statement is OK?

If there are any other reason please give me with easy language to understand.

N. B. If the question is not clear, please comment.

  • $\begingroup$ (everyone: please be nice) $\endgroup$ Dec 7, 2017 at 1:33

1 Answer 1


One of my teacher said that the one with more Molecular mass will deviate more from behaveing as ideal gas and he showed a graph given below

Yes, that's basically correct. But it has to be understood as a general trend, to which many exceptions can be found (especially when the Molecular Masses are close).

Ideal gases would be made up of point masses (taking up no volume at all) that collide perfectly elastically.

Real gases are made up of real masses that take up some volume (just condense them to see that) and substances with higher Molecular Mass (MM) tend to have larger molecules that take up more volume.

Secondly, higher MM usually means larger electron clouds surrounding the nucei. During collisions, due to transient polarisation (the electron clouds repell each other, remember?), there's more interaction (Van der Waals forces) between larger molecules. Larger molecules tend to be more polarisable.

So by and large, smaller molecules/atoms behave more ideally.

  • 1
    $\begingroup$ I think you should explicitly state that the teacher made a sweeping generalization for which numerous exceptions could be found. So generalization holds well if $m_1 >> m_2$ but not if $m_1 \approx m_2$ $\endgroup$
    – MaxW
    Dec 6, 2017 at 18:11
  • $\begingroup$ @MaxW: I think the question is about trends, at least that's how I read it. $\endgroup$
    – Gert
    Dec 6, 2017 at 18:12
  • $\begingroup$ @Gert yeah i remember about polarization $\endgroup$
    – user55439
    Dec 6, 2017 at 18:17
  • $\begingroup$ Added comment about trend. $\endgroup$
    – Gert
    Dec 6, 2017 at 18:20
  • $\begingroup$ @Gert - I understand. $\endgroup$
    – MaxW
    Dec 6, 2017 at 18:22

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