2
$\begingroup$

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.

Edit after a comment:

The abstract of this paper by B. P. Leonard express my concerns in a more elegant way: https://iopscience.iop.org/issue/0026-1394/47/3

|improve this question
$\endgroup$
  • $\begingroup$ 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) $\endgroup$ – M. Farooq Mar 23 '19 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 Mar 23 '19 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. $\endgroup$ – M. Farooq Mar 23 '19 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 Mar 23 '19 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. $\endgroup$ – M. Farooq Mar 23 '19 at 17:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.