Monday, October 6, 2014

Get rid of the mole?

From this week's C&EN and the Too Logical To Happen file:
Regarding the ACS Comment “Check Your IQ on the SI”: In theory, we can dispense with the mole (C&EN, Aug. 4, page 32). The mole, molarity, and molality were adopted as convenient ways to express relative amounts and concentrations of substances when we didn’t know the actual mass of atoms and molecules—when the very existence of atoms and molecules was in dispute. 
Now we know, which has enabled us to come up with the number 6.022 × 1023, give or take, and requires us to explain to students—who question why anyone would specify such a ridiculous number— that it wasn’t chosen but is experimentally derived. Now that we know the actual mass of atoms, ions, and molecules, we can describe the actual number of entities directly—for example, an acid as 1.5 YH+ per liter (Y = yotta = 1024), which is about 2.5 mol H+/L. We could rid ourselves of an unnecessary and confusing-to-beginners concept, but of course it will never happen. 
Howard J. Wilk
Philadelphia
Look, if we go away from the mole, what happens to all the cute mole stuffed animals and such?  

14 comments:

  1. Measurements are made to make it easier for people to do stuff. Does counting atoms rather than moles make it easier to do anything? If I'm a nuclear chemist, then counting atoms makes sense, because that's the scale I would be working on. Most chemists, though, use grams or mg or material, and counting in units of 1e24 or 1e21 atoms/molecules doesn't really help them - it just adds lots of spaghetti to the math. So why do it? Does it make it easier to count equivalents? No - you'd have to have the molecular mass (which would be the molecular mass - the mass of each molecule), and it would be in rather small units. Hope you like exponential notation....

    If changing a measurement system doesn't provide a significant intellectual or practical benefit, then you're making a rather large and disruptive change for little benefit. Since there isn't any more truth to be had from atoms than from moles, I can think of lots better ways to waste time and money.

    Maybe I should have recommended him to my cable provider - sounds like he needs a better hobby.

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    1. "Does counting atoms rather than moles make it easier to do anything?"
      That's what grams, milligrams, and picograms are for. The mole is simply no longer necessary for measurement. Why is this so hard for people to grasp?

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  2. Experimentalists will use any units they like, units for the sake of convenience - for example Torr.

    In 40s, nuclear physicists invented a neutron crossection unit barn, defined as 10^-28 m^2

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  3. Yes, get rid of Mole so we can dumb down the students even further.

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    1. They won't be dumbed down, they'll be allowed to "self-actualize"...

      Delete
  4. The difficulty here is the number of extant quantities expressed as "per mole." Eliminate the mole, and you open the Pandora's Box of "per what?". I actually disagree with JL's comment, as using "per mole" in many cases obscures some important conceptual points, and speaking of specific chemical entities will clearly complicate the situation, rather than dumb it down. Take a free energy change in kilojoules per mole, for example. If stoichiometric coefficients aren't all 1, we don't really mean "per mole of each reactant," we mean "per mole of reaction events."

    I would actually love to talk about reaction events, and have explicitly introduced things like extent of reaction to make teaching kinetics and thermodynamics easier. I do think too many chemists use "per mole" willy-nilly without really thinking about "per mole of what" (thank God our ancient experimentalist ancestors had the good sense to define some standards). However, if all you're going to do is replace "mol" with a metric prefix, what's the use?

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    1. When you write a thermodynamic value for a reaction, aren't you assuming that the reaction goes to completion (the per mole of reacted substance part)? If you want to know what happens under some set of experimental conditions (what reaction extent is getting at) rather than what happens when a mole of compound reacts, you need to be using a different tool. To subdivide reaction energies into useful quantities, isn't the mole a reasonable unit?

      Moles weren't handed down from your deity - they're just a convenience. Most of the time, they're OK, but if they get in the way, then you can use something else. I don't think junking the mole leads to an improvement in clarity or utility.

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  5. Damn - sucks to be a chemist these days. Sounds like "Milli" will be looking for a job soon: http://cenblog.org/newscripts/2011/09/beaker-vs-milli-mole/

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  6. Are we going to need calculators with 25 digits? Sigh...

    In principle we can get rid of any derivative units like Joules or Watts and use monstrosities like kg x m^2 x s^-2. However, science is not limited to just calculations. We have units to get a sense of scale and "shape" (properties) of what we observe. This ties our intuition to the computing part.

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  7. Astoundingly dumb idea. For one thing, it's far easier to visualize and manipulate moderately-sized numbers of moles rather than number x 10exp23.

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  8. I agree, what a bunch of mole-arky

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  9. No more "Kiss Me, I'm a Moles-ter" t-shirts.

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  10. Business opportunity: New mascot = "Yotta the Otter". Probably would go over better in Boston.

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