[impaler]

Hi all

I'm a chemical engineer so please go easy on me 

I've been trying to calculate molecular diameters for gaseous O₂, H₂O, SO₂ and SO₃.  To start with, I tried using equations based on viscosity such as this one, but for some reason the values are always 2-3 times larger than I expected.

Is it true that I can calculate a molecular diameter from the "second virial coefficient" and if so, how would I go about it?

Thanks in advance for your *delete me*

[FeLiXe]

with mean free path you get the effective collision radius
it is bigger than the atomic radius the or van der Waals radius

maybe that is the problem