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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: DougD on December 09, 2011, 06:20:45 PM

Title: Basis for mole definition.
Post by: DougD on December 09, 2011, 06:20:45 PM

After reading the mole is based on the number of particles as there are atoms in 12 grams of Carbon 12, I was curious about why 12 grams and not 1 gram.  Does anyone know the basis of this relationship?
Thanks,
Doug

Title: Re: Basis for mole definition.
Post by: Borek on December 10, 2011, 02:47:10 AM

Just for a convenience - this way molar mass in grams equals atomic mass in au.

Title: Re: Basis for mole definition.
Post by: Arkcon on December 10, 2011, 07:03:31 AM

Originally, we started with Hydrogen as equal to 1, under that scenario, carbon is 12.  Then we switched to carbon.  Like you said, it is all arbitrary, anyway.

Title: Re: Basis for mole definition.
Post by: juanrga on December 13, 2011, 02:06:20 PM

Quote from: DougD on December 09, 2011, 06:20:45 PM

After reading the mole is based on the number of particles as there are atoms in 12 grams of Carbon 12, I was curious about why 12 grams and not 1 gram.  Does anyone know the basis of this relationship?
Thanks,
Doug

Effectively, as other said it has a historical origin.

It must be worth to comment that the International Committee for Weights and Measures is studying changes to the definitions of the SI base units. Probably the future definition of the mole will not use ¹²C neither any other substance.