Talk:Molar mass constant

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This article is spurious: the "constant" discussed here is not a physical property or a fundamental constant, but instead a result of the definition of the mole combined with the (extremely common) practice of omitting the mass units used to define the mole.

The usual definition of the mole is the number of atoms in 12 g of carbon-12. Truthfully, this actually defines the gram-mole, or g-mol. There are other definitions of a mole, such as the kilogram-mole (kg-mol, equivalent to 12 kg of carbon-12) and the pound-mole (lb-mol, equivalent to 12 lbs of carbon-12).

Even though these definitions are confusing, this article only serves to make that confusion worse. The units of the molar mass constant (as the author of the article puts it) are, quite simply, 1/mol. In the case of defining mass in kilograms and "amount of substance" in g-mol, you must multiply by the conversion factor from kg to g (1000 g/kg), which is not a fundamental constant but a unit conversion. Kaiserkarl13 (talk) 22:38, 12 January 2009 (UTC)[reply]

The article is not spurious at all, and answers all the points you raise. Physchim62 (talk) 23:50, 12 January 2009 (UTC)[reply]

Actually, this article merely describes a constant that is created solely for the purposes of unit conversion. This is akin to writing Newton's second law as F = C m a, where C is the "force constant," having units of 1 N-s^2/(kg-m^2). This constant is not a fundamental constant of nature, but rather the definition of the Newton. The fact that the kilogram is used as the SI base unit of mass and the gram is used to define the mole in no way necessitates the use of a special constant to cancel the factor (0.001 kg/g) that results. Kaiserkarl13 (talk) 22:47, 20 January 2009 (UTC)[reply]

As a chemical engineer, you should see that your argument is not dimensionally homogeneous. The molar mass constant is fixed at a convenient value by the current set of definitions in the International System of Units. If the mole were to be defined in terms of a fixed value of the Avogadro constant – as many people believe it is at present, and as it may well be in the future – the molar mass constant would no longer have a fixed value, but instead a measurement uncertainty. The present situation is no different from having the speed of light defined by the definition of the metre (or maybe you're suggesting that, for this reason, the speed of light is not a constant either). Physchim62 (talk) 23:02, 20 January 2009 (UTC)[reply]

The value displayed in the page is wrong. It should be 1.66e-27 kg/mol. Hanselda (talk) 08:08, 17 December 2009 (UTC)[reply]

From May 2019 on, the value for the mass constant, calculated from measurements (including indication of uncertainty), should be displayed as a decimal number. 2A02:908:4B15:DA60:B59C:4204:BC8A:5884 (talk) 12:54, 25 November 2018 (UTC)[reply]