# How to calculate a rough estimate for the volume of a nitric oxide molecule?

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.

• The size of what? A molecule? A certain amount of molecules? – HDE 226868 Feb 10 '15 at 22:20
• Though this seems very clear to you, it's ambiguous for us. Details need be added. But anyways, welcome to chemistry.SE! – It's Over Feb 10 '15 at 22:30
• What details should I include? I'm trying to calculate a rough estimate for the size (volume?) of a Nitric Oxide molecule. – user13330 Feb 11 '15 at 0:28
• NO is in equilibrium with weak dimer form. May be important. – permeakra Feb 11 '15 at 3:32

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.
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 $\pu{3.58\times10^-29 m3}$.