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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: aerosam on May 12, 2012, 07:48:43 PM

Title: Determining molecular formula of a compound
Post by: aerosam on May 12, 2012, 07:48:43 PM

Hi

Suppose i have an unknown inorganic compound, is there a way to determine it's molecular composition.
For example could a mass spectrometer be used? If not which other device should i use? How would the presence of small amounts of impurities affect the results?

Thanks
Sam

Title: Re: Determining molecular formula of a compound
Post by: gelminil on May 15, 2012, 04:44:06 PM

I suppose a mass spectrometer could be used.
usually you can do this gravimetrically.

If you know it has Ba2+ and Cl- ions.
You can precipitate the Ba2+ with sulphate ions and the cl- with silver nitrate.  Compare the moles of each.

Title: Re: Determining molecular formula of a compound
Post by: discodermolide on May 21, 2012, 05:47:39 AM

Quote from: aerosam on May 12, 2012, 07:48:43 PM

Hi

Suppose i have an unknown inorganic compound, is there a way to determine it's molecular composition.
For example could a mass spectrometer be used? If not which other device should i use? How would the presence of small amounts of impurities affect the results?

Thanks
Sam

A MS will only give you the molecular weight of the compound plus the weight of fragment ions. I suppose you could piece it together but you will probably get the wrong answer. Impurities will make the MS harder to interpret-
The best way is elemental analysis. Here impurities will significantly affect the results.
NMR and IR will help a lot in this respect, the former will tell you what functionality is present, the latter what functional groups are in the molecule.

Title: Re: Determining molecular formula of a compound
Post by: discodermolide on May 28, 2012, 06:24:03 PM

Quote from: discodermolide on May 21, 2012, 05:47:39 AM

Quote from: aerosam on May 12, 2012, 07:48:43 PM

Hi

Suppose i have an unknown inorganic compound, is there a way to determine it's molecular composition.
For example could a mass spectrometer be used? If not which other device should i use? How would the presence of small amounts of impurities affect the results?

Thanks
Sam

A MS will only give you the molecular weight of the compound plus the weight of fragment ions. I suppose you could piece it together but you will probably get the wrong answer. Impurities will make the MS harder to interpret-
The best way is elemental analysis. Here impurities will significantly affect the results.
NMR and IR will help a lot in this respect, the former will tell you what functionality is present, the latter what functional groups are in the molecule.

Please ignore my comment concerning NMR and IR, I read organic for inorganic, apologies.