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1 Mol = SI Unit - What the ?

  1. Oct 31, 2008 #1
    1 Mol = SI Unit - What the...!?

    This is ridiculous. How can 1 Mol be considered an SI-BASE-UNIT although it is simply a factor

    1 Mol = 6.022 * 10²³

    Every attempt to explain this thorough misconception is naturally predestined to fail! I'm not looking for an explanation, neither going to discuss that - I'm looking for an EXCUSE to this absurd claim!

    (you know how to take that last of my senteces)
     
  2. jcsd
  3. Oct 31, 2008 #2

    mgb_phys

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    Re: 1 Mol = SI Unit - What the...!?

    What else would the units of amount be?

    You could argue that it should be 6.022E26 since it would be the number of atoms in 12Kg of C12 rather than 12g
     
  4. Oct 31, 2008 #3
    Re: 1 Mol = SI Unit - What the...!?

    THE UNIT OF "AMOUNT" IS "1"

    For the sake of... Mol is not an SI-Unit just because the chemists say so!
     
    Last edited by a moderator: Oct 31, 2008
  5. Oct 31, 2008 #4
    Re: 1 Mol = SI Unit - What the...!?

    Look, as a matter of fact, if "Mol" was an SI-Unit this would naturally exclude "1" from being an SI-Unit. WHICH, of course, cannot be true (you don't want to argue that, do you?).
     
  6. Oct 31, 2008 #5

    mgb_phys

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    Re: 1 Mol = SI Unit - What the...!?

    No problem, 1 is just 0.4 yottaMoles !
     
  7. Oct 31, 2008 #6

    berkeman

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    Re: 1 Mol = SI Unit - What the...!?

    Could you please provide a pointer to an mks/SI referencd that discusses moles as a fundamental unit? I'm interested in reading about why it would be fundamental as well. Seems possibly useful, but it would require a good explanation....
     
  8. Oct 31, 2008 #7
    Re: 1 Mol = SI Unit - What the...!?

    Will I get banned for quoting Wikipedia up here? Oops, already mentioned it... You got my reference :P

    mgp_phys: 1 has to be an SI-Unit BY DEFINITION. If 1 WAS NO SI-Unit you could not use scalar values in any SI-phrased expression, because every scalar has the unit of "1". Please, thats not open for discussion. 1 is an SI-Unit, even though it might not be listed among the Sys. Intl. units, it still is.
     
  9. Oct 31, 2008 #8

    f95toli

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    Re: 1 Mol = SI Unit - What the...!?

    I think you have misunderstood what the purpose of the SI.

    Whether or not a unit is "fundamental" or not has nothing to do with it. SI is first and foremost designed to be a system of practical units that can be used to perform traceable calibrations of measurement equipment for science and industry. This is why e.g. the candela is also a base unit in the SI.
    So while it would make "physical" sense to use "1" as the base unit it would not be practical: there would be no way of performing traceable calibrations.
    I.e. it is the same reason why we still use an artifact to define the kilogram; in theory you could just defined 1kg= a certain number of atoms of a specific isotope. However, this unit would be impossible to realize.

    Edit: Btw, "1" is not in the SI.
     
  10. Oct 31, 2008 #9

    mgb_phys

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    Re: 1 Mol = SI Unit - What the...!?

    Not impossible, just a bit of effort. It's nearly there http://www.acpo.csiro.au/avogadro.htm
     
  11. Nov 1, 2008 #10

    rbj

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    Re: 1 Mol = SI Unit - What the...!?

    the excuse is that although a mole turns out to be a not-so-certain count of elements (and that count is NA), we are not certain precisely what that number is, because of the real definition (not yours above) of the mole. the mole is defined to be the number of Carbon-12 (12C) atoms that make up precisely 12 grams of mass. because, given the pre-existing definition of the gram (or kg), in terms of that definition we don't know exactly how much mass a 12C atom is, so we don't know exactly how many of those atoms make up 12 grams.

    we are, by definition, anthropological beings and have chosen units that are anthropocentrically convenient. it's no accident of people (although, from the POV of the aliens on the planet Zog, it would be an accident of history in our neck of the woods) that a meter is about as big as we are. that a second is about as long as a fleeting thought. that the kg is something we can pick up and hold.

    it is clear that SI defines at least 3 more "fundamental" units than are fundamental from the POV of physical quantity. i doubt that there will ever be a "Planck mole" (if there is, i would suggest it should be the number of 1H atoms that make up a Planck mass). the Kelvin, Mole, and Candela are all unfundamental in that sense. essentially, the Boltzmann constant, Avogadro's number, and [itex](4 \pi)/683[/itex] watts (perhaps with some anthropometric brightness data) are all just expressions of those contrived unit definitions. or you could say the other way around; the Kelvin, mole, and candela are expressions of those constants, which are in no sense universally fundamental.

    IMO, there are four fundamental dimensions of physical "stuff": Time, Length, Mass, and Charge. SI might have defined the unit of electric current first, but since they already had a definition for the unit of time, it doesn't matter. so we human beings define units for time, length, mass, and charge (via current) completely anthropocentrically (let's put us back before 1960, when the meter was the length between two scratch marks on a platinum-iridium bar in Paris), then we go out and measure these universal parameters, c, G, [itex]\hbar[/itex], and [itex]\epsilon_0[/itex]. we could have instead, defined those values to be what seems convenient and then construct the units of time, length, mass, and charge. and we did that partially already, c and [itex]\epsilon_0[/itex] (as a consequence of the defined c and [itex]\mu_0[/itex]) are set to defined values because of how the meter and ampere are defined. the unit time is still set arbitrarily (but it uses a nice time base now and no longer the wobbly earth) and the unit mass is still a prototype (a thing).

    there is a lot of discussion to redefine the kg away from the prototype and, similarly to the meter and c, in such a way as to fix a dimensionful universal constant. the leading candidates are NA and [itex]\hbar[/itex]. they can redefine the kg to set one of those to a fixed constant (like is done for c) but not both. these are competing proposals and i don't know which one will win, but i hope the watt-balance definition (that fixes [itex]\hbar[/itex]) is the one that wins.
     
  12. Nov 2, 2008 #11
    Re: 1 Mol = SI Unit - What the...!?

    As far as I can tell the mol is considered a base unit because it is a unit of measurement that cannot be derived from any other base unit that is used in physical and or chemical measurements. As far as I can tell any measureable quantity (in the SI) that cannot be expressed mathematically by combining any of the other SI base units qualifies as a base unit regardless of what it is (even if it is a Candela and nobody uses it...).
     
  13. Nov 2, 2008 #12

    rbj

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    Re: 1 Mol = SI Unit - What the...!?

    the candela really just boils down to an anthropometric definition of radiant power (4[itex]\pi[/itex]/683 watts) at a given frequency (540 x 1012 Hz, i think that's around the color Yellow) and that's supposed to define a unit of perceived luminance. sorta like defining absolute decibels. the candela is not defined by SI for other frequencies, so to try to attach a meaning of candelas of luminosity at other colors (or frequencies), you need perceptual data like this. then a candela is something other than 4[itex]\pi[/itex]/683 watts, but this is not part of the SI definition. human perception of physical quantities is not the same as the physical measurement of physical quantities. the Kelvin is just an expression of the Boltzmann constant (or the other way around), the mol is just an expression of the accepted mass (in kg or whatever units we like) of the 12C atom, the candela is just an expression about how sensitive the average human eye is for yellow colored light. none of these would need to be standardized units for physical quantity but could, instead, be part of the published standards and practices of various applied sciences. dunno why SI would need to be part of it.
     
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