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 expresses my concerns in a more elegant way: Metrologia 2010, 47 (3), L5–L8

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.

  • $\begingroup$ Check the articles in Metrologia (An official society journal dealing with measurements) with key words mole and kilogram. None of the approaches are problem free. See for example, "Comments on recent proposals for redefining the mole and kilogram" by BP Leonard (2010) in Metrologia. One the Avogadro number is redefined, the C-12, will no longer remain 12.00000000000000000 (ad infinitum) $\endgroup$
    – ACR
    Commented Mar 23, 2019 at 16:25
  • $\begingroup$ Thanks. I am reading already materials that I found links for here. One point is that we already approximated things all along. For instance, taking an amount in g equal to the molecular mass assumed 1 atomic mass unit to be the same, while is not as every atom has is own mass defect. Ie. We never got an exact number of Avogadro of particles, even in principle. That was for 12 C only. I am always surprised by the depth, practical and experimental, of the reasoning involved in defining units. $\endgroup$
    – Alchimista
    Commented Mar 23, 2019 at 16:34
  • 1
    $\begingroup$ True, the atomic masses are not absolute either. I have asked at so many places as to why we don't measure absolute atomic masses, and nobody has an answer including myself despite having decade long experience in the academic/research world. The choice of C-12 is also arbitrary because the true mass of C12 cannot be 12 exactly. A reasonable answer was it that it experimentally difficult to measure absolute masses. The problem is that from schools we start to take all these units for granted and never give a second thought and nobody (teachers) ever bothers to make us give it a second thought. $\endgroup$
    – ACR
    Commented Mar 23, 2019 at 17:03
  • $\begingroup$ But I am not in school and I consider myself a chemist, so I am upset with myself. All this started because explaining the absurdity of homeopathy. My friend asked about Avogadro number and mole. While this was ok, I afterwards realizes starting May something is deeply different (perhaps no but so it appears to me). Anyway I will edit my Q linking to the Leonard abstract that renders my concern in a more elegant way. @M. Farooq $\endgroup$
    – Alchimista
    Commented Mar 23, 2019 at 17:12
  • $\begingroup$ @ Alchimista, I was sure that this query came from a chemist not a student. My interest also arose because my niece was asking about Avogadro and mole. After explaining the typical story, I had a moment of guilt as I felt I just repeated what was written in her school book. $\endgroup$
    – ACR
    Commented Mar 23, 2019 at 17:46


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