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[Leafsnail]

I always found it weird that a kilogram is SI but a kilometer ain't.

Yeah - you'd kindof expect them to call the SI one grams in the first place.  It's also weird because it's defined by an artifact (so you have a block of metal that is the kilogram) rather than a constant.

Actually the mole is derived from mass.

It involves "kilogram" in its definition, yeah, but it still needs a definition rather than just "Unit X divided by unit Y", so it's a base unit.

[Darvi]

I always thought a mole was 1/mass of a proton.

[Leafsnail]

"A mole is a number of particles equal to the 1/12 of the number of atoms in 12g of carbon-12" is the definition I have on tap for my current chemistry test .

I guess 1/mass of a proton is basically it though, although the fact you have to bring mass of a proton into it at all makes it a base unit.

[olemars]

That's incorrect. One mole is the amount you have of a substance if you have a number of particles of it equal to avogadro's constant. The formal definition is the number of atoms in 12g of carbon-12. So you have one mole of something if you have 6.02214078×10^23 particles of it.

[Darvi]

Yes, but that constant got defined using mass, right?

Actually, wikipedia says so too.

[olemars]

Sure, but the mole as a unit is just a count, basically multiples of avogadro.

[Muz]

The Urist, obviously.

Haha, I probably missed out some meme around here, but I remember when some of us B12ers got onto a RPG and used Urists to measure tiles.

BTW, favorite unit is Kelvin. So elegant.