Why there is a need of independent definitions for both mass and mole?

Considering that the two quantities are linked by reality, how can be both imposed by independent definitions?


I am aware that the change taking place next May does not effect common weighing in chemistry and engineering, but still the new definition of mole (an exact number of entities, to be called Avogadro number as well) does confuse me, *conceptually*.


Direct counting is not a practical way of sampling, so already I do not see the good of defining mole as a fixed number of entities (of course I have thought in this way already, but I am not a definition :).


How can I now weigh a mole of substance, in principle? It seems that without a history-of-science background, armed by the mass definition and mole definition, I cannot weigh in moles despite of having both very well defined.


Before, the "only" knowledge required was a table of atomic masses, and one could have done that even ignoring the value in grams for 1 amu, if not interested in spelling out the value of Avogadro number itself. I think is what happened before mass spectroscopy sort of techniques revealed the absolute mass of particles and atoms.


To date, the Mole is conceptually and mathematically linked to a mass measurement, ie. the amu and its multiples. With the new definition the two things are distincts, as 12 g of 12 Carbon is not a Mole anymore.


Please clarify without listing the non-operational pros of the new definitions, as the fact that the latter stresses the difference between matter as mass and matter as substance even more than before, to mention one that, as a chemist, I like much.


The abstract of this paper by B. P. Leonard,[*Metrologia* **2010,** *47* (3), L5–L8](https://doi.org/10.1088/0026-1394/47/3/L01) expresses my concerns in a more elegant way: [*Metrologia* **2010,** *47* (3), L5–L8](https://doi.org/10.1088/0026-1394/47/3/L01)



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> The fundamental concept of the mole requires the number of entities comprising one mole, i.e. Avogadro's number, to be exactly equal to the gram-to-dalton mass ratio. If this compatibility condition is to be satisfied, the mole, the kilogram and the dalton cannot all be defined independently. This note concerns recent Metrologia publications that do, however, propose independent definitions of all three quantities: the mole by fixing the value of Avogadro's number and the kilogram by fixing the value of the Planck constant, while retaining the current carbon-12-based dalton. Adoption of these incompatible definitions would likely cause serious widespread confusion and might even result in a split in scholarly and technical communication between the quantum
> physics and chemistry communities. Other entirely compatible alternatives are possible: either retaining the current (inexact) carbon-12-based mole and dalton with an independently redefined kilogram or redefining the mole by fixing the value of Avogadro's number, with a compatible dalton that is exact in terms of the redefined kilogram.
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