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(solved) Need a Volunteer : Science : How do they guess the chemical composition of things ? Answered

Hello :-) Here is one of my many unanswered questions ... I googled a lot about this one, but so far, I failed to find anything usefull, probably because I don't have the correct vocabulary. That's why I'm asking to you ... I'm still wondering how scientists manage to guess the chemical composition and molecular structure of things ... For example, how do they know that saccharose is C12 H22 O11, and that it's molecular structure is like the picture below ... About crystals, I've read that they use Crystallography to determine the arrangement of the structure ... But how do they do with organic elements ??? Thanks in advance for your help :-) PS : I can't post this thread into the "Volunteer" group forum. It says that I don't have the autorisation. Do I have to join the group too ?


They don't guess y'know.
Methods to Google are:
X-ray crystal structures (don't show hydrogen too well, but it's not hard when you know where the heavy elements are)
1H / 13C NMR spectroscopy
FTIR spectroscopy
UV spectroscopy
Then there are chemical transformations & subsequent analysis, which give indirect information.


I've done enough of this... For organics you tend to go with 1H NMR for a start, mass spectroscopy (MS) gives you molecular weights and fragments. IR & UV aren't going to help you much with sugars, and combustion-analysis isn't often much use as compared with MS. A good 13C NMR can help complete the job, but unless you're going for an X-ray crystal structure, you're largely deducing structure by comparison with other known molecules.
Also, structures can differ in solution (and be solvent-dependant) as compared to the solid state.


They don't know that's why everything taste like chicken :)

. For one thing, the molecular structure diagram is a 2-D representation of a 3-D object - so it is inherently inaccurate. It's just a fancier shorthand than the C12H22... business.
. Googling molecular structure +angle turned up this at Wikipedia:
Molecular geometries are best determined at temperatures close to absolute zero because at higher temperatures the molecules will show considerable rotational motion. In the solid state the molecular geometry can be measured by X-ray crystallography. Geometries can be computed by quantum mechanical calculations or by semi-empirical molecular modeling.

( Oh, seems I missed this article ... probably because it does not have any french translation ... :-} I'm forced to "juggle" between each languages if I want to find more information with Wikipedia ... )

So, yeah, at 0°K, everything is solid and stable enough for a crystallography. That sounds almost obvious finally ...........

Thanks NachoMahma for your help :-)

BTW, I've just found (at last) how to ask to Google about "how do they guess the chemical composition of things" ... It's simply : "Elemental analysis"
wikipedia - elemental analysis
So, seems I finally have all the answers I was looking for about this topic =o)
thanks again