Redefining the Mole
nist.gov
Redefining the Mole
1–10 of 30 posts
Re: Redefining the Mole
#2My daughter was asking me about moles recently. Glad to see NIST has something I can point her to.
Re: Redefining the Mole
#3The 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
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.
Re: Redefining the Mole
#4While 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.
Re: Redefining the Mole
#5> 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.
Re: Redefining the Mole
#6The 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 In particular: The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.022140857…
I remember struggling with moles as a concept in school up to the point when someone finally told me that "a mole" is not like "a kilogram"; it's like "a dozen".
Re: Redefining the Mole
#7> 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"…
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).
Re: Redefining the Mole
#8Re 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.)
Re: Redefining the Mole
#9> 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"…
The purpose of the mol is to be a convenient measure for us working in the SI unit off grams. We need to get between grams and a count of molecules, that's the number.
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).
Re: Redefining the Mole
#10The 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 In particular: The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.022140857…
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).