[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.

[cowsandmilk]

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).

[rasteau]

>We need to get between grams and a count of molecules, that's the number.

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.

[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.

[nkrisc]

Also consider you're unlikely to have a 100% pure sample of whatever it is you're measuring anyway, at least if it's a large enough sample to hold in your hand.

Approximations are still useful, even if they're not good enough for all applications.

[zeveb]

But that's a circular definition; one could use a different unit than grams (e.g. grains) and get a different pseudo-mole and hence a different pseudo-Avogadro's number.

[syrrim]

>"how is the mass number defined?"

>"that's easy! we take the mass of one mole of atoms, and that's the mass number!"