Talk:Molecular mass
One i saw molecular mass crossing the street, i spend up and maimed him. He dont come round these part no more.
Although relative molecular mass COULD be considered dimensionless, it is more widely quoted in the units of mass per unit amount (in SI, kg/kmol), where the amount of a substance is given by the total number of molecules divided by Avagadro's constant (approx 6 times ten to the power twenty three).
I move that this be struck from the list of dimensionless numbers on this site, as molecular mass (by it's very name) is not non-dimensional.
- I concur. It is only with an additional modifier such as relative molecular weight that it could even be considered dimensionless. Gene Nygaard 02:49, 6 Feb 2005 (UTC)
Molar mass in SI
It is commonly beleived that in SI the molar mass of a substance is the mass of one kilomole, therefore it has the unit kg/kmol. This is against the very basic principle of SI. The unit of chemical substance is mol (defined as containing Avogadro number molecules), the unit of mass is kg, so the unit of molar mass is kg/mol (for instance 0.01802 kg/mol for water.) The molar mass of any substance can be obtained multiplying the relative molecular mass (often still called molecular weight and abbreviated as MW) by 0.001 kg/mol.
A similar mistake is often made in SI when giving the universal gas constant. Its unit should be J/(mol×K) that is Ru = 8.3145 J/(mol×K) is the correct specification and the use of kmol should be avoided!
It is not erroneus to give Ru = 0.0083145 kJ/(mol×K) though, because any unit can be multiplied by a multiplier kilo and the multiplier can be concatenated with the first term in the numerator. (However, it is difficult to see any advantage of the multiplier in this case.)