Redefining the Mole
nist.gov
nist.gov
Every example I find describing the utility of the mole could just as plausibly substitute "dozen" or "googol" for "mole". I'm not clear on what would be lost to science by instead declaring a new number that is untethered from Avogadro's historical dependence on mass or length. Perhaps the deeper issue is that I'm not clear on why the dimensionless mole is a base unit at all.
Sure, we could work in dozens, but then we would have some other arbitrary constant we would have to memorize to go from grams to a count of molecules. And the nice thing about mols is that you don't have to memorize Avogadro's number to use them; while you would have to memorize a constant to go from grams to any other unit of counting molecules (that constant in the case of mols is 1, it would be something else for any other choice of units).
I think that number is only true for carbon-12.
>but then we would have some other arbitrary constant we would have to memorize to go from grams to a count of molecules.
I think we already have to do that, hence molar mass.
Speaking as someone who has had to deal with rounding errors in floating-point graphics, data structure layouts, and real estate cartography, that sounds horrifying and insane.
The reason it cannot be exact for all atoms is forced on us by nature: atoms come in isotopes, each weighing slightly differently, and most common elements come in a mix of isotopes.
So picking one isotope of one element (carbon-12) as the definition for a mole that is decently representative of how chemists will use the number is a perfectly fine and useful number.
Any chemist that needs to worry about the fuzz will understand this and act accordingly. For example, carbon 22 has a mass slightly larger than 22/12 that of carbon 12 (but has short half-life). Carbon 13, which is stable, has mass slightly over 13/12 that of carbon 12, and when using it, one adjusts accordingly. And these "slightly over" phrases are also known to many digits of precision.
But nailing down the number precisely is extremely useful.
Approximations are still useful, even if they're not good enough for all applications.
>"that's easy! we take the mass of one mole of atoms, and that's the mass number!"
For others with the same nagging thought, the explanations of Avagadro’s Law feel more familiar: https://www.britannica.com/science/Avogadros-law
In particular:
The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.022140857 × 10^23, a quantity called Avogadro’s number, or the Avogadro constant. For example, the molecular weight of oxygen is 32.00, so that one gram-mole of oxygen has a mass of 32.00 grams and contains 6.022140857 × 10^23 molecules.
But, when I ask for the exponent, they are really quite uncertain. For many, the rote learning has cut off after the "times ten to the" in the sentence. This is disappointing, but at least it provides me an opportunity to talk about what digits actually mean.
If I were a psychologist (shudder) interested in how learning works, I'd be inclined to see whether the students who get exponents wrong are the same as those who have no idea how to handle significant units. My guess is that they are. I think the problem is that the core ideas of decimal notation are lost on many learners, because all digits are equal on a calculator, so getting the "2" wrong in Avogadro's number seems to be the same as getting the "6" wrong.
If I had a magic wand to wave, I'd use it to bring back slide rules. (Oh, and I'd bring back low grades for weak work, but that also would not fly with school boards.)
Atoms in 1g
Of 12C?
It's 6E 23.For instance you might have wondered how much weight you lose with each breath. Converting O2 to CO2 means you're losing some mass of carbon each exhalation and that's a major channel for weight loss.
But not remembering much about chemistry, nothing comes of this. Another part of our lives remains shut off because its easier to ignore it.
Not terribly important I guess. But add up the thousands of times we move ahead without real information or curiosity, and our lives are diminished.
No, I'm not ready to be proud of how I lost most of my technical knowledge about the world, and how I blunder on in ignorance because its easier.
Indeed, that's almost 100% of weight loss, except for some daily fluctuations like water weight and what happens to be inside your gut at the moment, right? That's virtually the only way that body actually loses its own mass.
Interesting to find out that the concept every chemistry student is taught is about to be redefined. Odd that they couldn't schedule it on mole day...
I remember struggling with the concept of the mole in high school.
After working many problems I was able to see what the big deal was about:
The mole links the macroscopic world we can directly experience with our senses to the atomic world which we cannot.
Think of the mole as a monetary exchange rate between these two worlds. It converts mass of a pure sample (which we can measure directly on the bench top) to number of particles (which we can't). Chemistry and accounting have a lot in common. If you're good with money, you should be good at chemistry.
Anyone can pick up an ingot of silver, place it on a balance, and read the number to get the mass. Use of the mole (and the atomic weight of silver) allows this measurement to be converted into the number of silver atoms in the sample. This process is identical to the one you'd use to figure out how much your hotel in Paris will cost you in dollars.
I don't think it's quite that simple, but I can see where you're coming from.
Neither units of money nor atoms can be created or destroyed. We use math to figure out what happened in a transaction or reaction.
Still, I'm curious - where do you see a fundamental divergence between the two?