Wouldn't it be possible to identify a chemical by measuring its molar mass? How much accuracy does the measured molar mass need to be to guess what chemical it could be?

EDIT: @ortho makes an important point about isomers, but organic chemistry is much harder ;). Let's just focus on inorganic chemistry (though there are still isomers, usually there aren't).

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    $\begingroup$ To some extent it is, for example you can look up high-resolution mass spectrometry. chemguide.co.uk/analysis/masspec/mplus.html However if you only have the molar mass there's no way you can differentiate between isomers (e.g. n-butane and isobutane) $\endgroup$ – orthocresol Oct 18 '15 at 21:50
  • $\begingroup$ It’s not like there be no isomers in inorganic chemistry, see cis-diammindichloridoplatinum and trans-diammindichloridoplatinum. $\endgroup$ – Jan Oct 18 '15 at 22:01
  • $\begingroup$ @Jan i know, but usually there are no isomers, while organic chem is much different.. $\endgroup$ – TanMath Oct 18 '15 at 22:02
  • $\begingroup$ Sorry, wrong... $\endgroup$ – Jan Oct 18 '15 at 22:02
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    $\begingroup$ CO and N2 has same molar mass. There is a bunch of other examples as well. $\endgroup$ – permeakra Oct 18 '15 at 22:33

It depends.

This was pretty much the way that the elements were discovered.

For inorganic chemicals in general the problem is really one of specificity. So I want to work with a chemical sample between 1 gram and 20 grams and I have an analysis good to 1%. That only yields about 300 statistically significantly different numbers $1.01^{300} = 20$. So for various inorganic salts this wouldn't work too well.

To take this into another field think of geology. Hardness and specific gravity are simple tests used to help identify minerals. Neither is very good by itself, but combined they sort out the minerals into a lot more bins.

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