I really can't understand why is the mole a unit of measurement and not just a constant. I mean, we use units to express things that we can measure but that are not countable, that are continuous - and not discrete. However, the number of atoms (or whatever) of something is clearly countable, discrete. It is also a huge number, which would request for a constant, a number to multiply to: Avogadro's constant is that number, like mega or kilo in SI for other units. We could still write 14 mole, where mole is the Avogadro constant (exactly the way it is), but every book I read so far states that the mole is a unit of measurement, official in the S.I., and so does Wikipedia. It just makes no sense to me that it is considered that way, but there is obviously a reason why... What is it?

  • $\begingroup$ I've been thinking about this question too! Why is "mol" a "Base SI unit" when "radian" is not? $\endgroup$ Jul 11 '20 at 23:12
  • $\begingroup$ Good question. It got even more confusing since: "The definition of mole was adopted in November 2018 as one of the seven SI base units" :-/ $\endgroup$ Oct 27 '21 at 14:44

I think you are confusing unit of measurement with fundamental unit. A unit of measurement is a definite magnitude of a physical quantity. Consider a very common grouping unit, the dozen which is conceptually the same as a mole and is a little easier for us to grasp since it is a commonly used small number. As far as I know, the only requirements for a unit of measurement to be valid is general consensus. If you and I agree that we will measure distance in units of 'Chemistry Textbook Thickness', then it is a valid unit of measurement.

A fundamendal unit is a part of a particular set of units upon which all other units can be derived. In this case, the mole is related to the fundamental unit of mass and is defined as an amount of items equal to the amount of carbon atoms in 12 grams of $^{12}C$.

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    $\begingroup$ Indeed, I think I meant fudamental unit. It just seems to me that it makes sense that the meter is a fundamental unit: you can't just say the distance between you am me is 20. It requires a unit. The mole, on the other hand, is exactly the opposite: it seems to be a unit to count things that do not require units do be counted, like atoms or potatoes. $\endgroup$
    – Luan Nico
    Jun 16 '13 at 17:06
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    $\begingroup$ Yet this does not answer the main question: why not simply consider the mole as a constant? A mole is certainly not a dimensional unit such such as one representing mass or time... It is just a number, somewhat like $pi$ or $10^{24}$. $\endgroup$
    – Oliver
    May 30 '14 at 6:57
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    $\begingroup$ Like @Oliver, I'm not sure this answer addresses the actual question. Nobody disputes that you could call a mole a unit, and indeed the SI system does, but the question is why does it? "Chemistry Textbook Thickness" is not comparable to moles or dozens, because that's defining a unit of measurement for a concept (length) that we have no other way of defining. When it comes to a simple discrete number of objects, though, the thing looks to me like it's unitless. It's simply a number, and multiplying it by 6E+23 and saying that's a unit doesn't seem logical. $\endgroup$ Apr 30 '17 at 21:18
  • $\begingroup$ "The definition of mole was adopted in November 2018 as one of the seven SI base units". So now, the mole is a fundamental unit, and does not depend on mass anymore. Which means the above question still stands, and your answer doesn't apply anymore. $\endgroup$ Oct 27 '21 at 14:44

Let me add to bobthechemist's answer. The mole counts quantity. The fundamental unit involved is Avogadro's Number, approximately $6\times 10^{23}$. So $6\times 10^{23}$ units of anything is a mole of that thing, whether it is atoms of hydrogen or sand particles.

With this in mind reread bobthechemist's answer.


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