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I was wondering what would be the best way to calculate a rough estimate for size of a nitric oxide molecule to use in comparison to a box.

I was thinking volume would be the best method, but this gave me an answer in dm3/mol which I can't use for comparing against the volume of a box.

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  • $\begingroup$ The size of what? A molecule? A certain amount of molecules? $\endgroup$ – HDE 226868 Feb 10 '15 at 22:20
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    $\begingroup$ Though this seems very clear to you, it's ambiguous for us. Details need be added. But anyways, welcome to chemistry.SE! $\endgroup$ – It's Over Feb 10 '15 at 22:30
  • $\begingroup$ What details should I include? I'm trying to calculate a rough estimate for the size (volume?) of a Nitric Oxide molecule. $\endgroup$ – user13330 Feb 11 '15 at 0:28
  • $\begingroup$ NO is in equilibrium with weak dimer form. May be important. $\endgroup$ – permeakra Feb 11 '15 at 3:32
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Take the volume per mole value (b) from a van der waals constants table (0.0289 L/mol) and divide by $6.022 \times 10^{23}$ molecules per mole.

This will give you a volume that you can compare to the box.

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  • $\begingroup$ This gives such a small number,10^-26. I was thinking it would be around 10^-15 or larger. Does this look correct to you? $\endgroup$ – user13330 Feb 11 '15 at 0:32
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    $\begingroup$ for one molecule, 10^-26 liters sounds much more correct than 10^-15 liters. The distance from the center of the oxygen atom to the center of the nitrogen atom is 115 picometers. $\endgroup$ – DavePhD Feb 11 '15 at 1:01
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On the MP2/def2-QZVPP level of theory the $\ce{NO}$ bond length is $\pu{113.5 pm}$ and the molar volume is estimated based on $\pu{0.001 e/bohr3}$ density envelope as $\pu{241.539 bohr3/mol}$ or $\pu{21.555 cm3/mol}$. The molecular volume therefore is about $\pu{3.58\times10^-29 m3}$.

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