
Redefining the Mole - nj65537
https://www.nist.gov/si-redefinition/redefining-mole
======
rasteau
> _In practical terms, the mole helps chemists measure stuff. It helps express
> the amounts of atoms or molecules in a chemical reaction. Cause a half-mole
> of oxygen molecules (O2) to react with a mole of hydrogen molecules (H2) and
> you get a mole of water (H2O)—equal to about 18 grams of substance._

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.

~~~
jpallen
It's an arbitrary number, but it's nice because one mole of atoms with atomic
mass number X will weigh approximately X grams. This is exactly true for
carbon-12 (and is what defines a mole).

~~~
a1369209993
> one mole of atoms with atomic mass number X will weigh _approximately_ X
> grams

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.

~~~
ChrisLomont
There is no other way. The problem is atoms combine in integer ratio amounts
to form molecules. To mix things for chemistry one needs to be able to mix
things in proper proportions. So having a number that trades number of atoms
to something plausibly measurable, like mass, is needed.

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.

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WalterGR
The definition of a mole provided in this article is no doubt correct, but
it’s not how it was taught in my high school chemistry class.

For others with the same nagging thought, the explanations of Avagadro’s Law
feel more familiar: [https://www.britannica.com/science/Avogadros-
law](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._

~~~
sesqu
The article does briefly discuss a slightly more accurate version of that
definition, and why it's bad (it's convoluted and relies on the poorly-defined
kilogram).

~~~
zaarn
The kilogram will get a better definition this year if nothing holds it up
(atleast, redefining it based on the planck's constant is planned for the 2018
meeting according to WP).

~~~
SamBam
Right, that's a main point of the article.

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saagarjha
While the article makes an excellent effort at making the material accessible
to the general public, I find it kind of sad that they automatically assume
that any sort of math is "cryptic" or "complicated"–i.e. something to be
avoided.

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bluenose69
Re the educational aspect -- when I ask students in my university class
whether they remember Avogadro's number from high school, they all respond in
the affirmative. When I ask them for the mantissa, the whole class sings out
"6.02". Great!

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.)

~~~
pbhjpbhj
6E23! It rhymes. If they can remember "6E" and that it rhymes, then the first
number after 3 they come to that rhymes is 23.

    
    
        Atoms in 1g
        Of 12C?
        It's 6E 23.

~~~
SamBam
...in 12g, surely?

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apo
_On November 16, 2018, representatives from more than 50 countries are
expected to make history when they gather in Versailles, France, to vote on
redefining the SI, including the mole. The vote will close the book on this
chapter of Vocke and Rabb’s work, but will open a new chapter in chemistry as
the fundamental unit, for that branch of science will no longer be tied to a
physical object but a constant of nature._

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.

~~~
WilliamEdward
>If you're good with money, you should be good at chemistry.

I don't think it's quite that simple, but I can see where you're coming from.

~~~
apo
I should have qualified that with "General Chemistry." You can solve the
majority of General Chemistry problems by looking at them from an accounting
perspective.

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?

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anticensor
The important redefinition of year is of _the kilogram_. Heck, IPK no longer
weighs one kilogram :)

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killjoywashere
My daughter was asking me about moles recently. Glad to see NIST has something
I can point her to.

~~~
mrec
So does XKCD: [https://what-if.xkcd.com/4/](https://what-if.xkcd.com/4/)

