5 ms·

An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a

An Avogadro's constant number of molecules is one mole. One mole of hydrogen has much less mass than one mole of iron. You have to choose an arbitrary mass of a single element as the base quantity. Hydrogen might be the best theoretically, it's just a proton and an electron, but it is tricky to work with because it's a gas at room temperature. So instead the mole has been defined as the number of molecules in a specific mass of carbon-12.

filmor 8y ago
Carbon 12 is being used because that accounts for the nuclear forces which are absent for H-1.
mbreese 8y ago
I think this is exactly the rationale. But, from a more practical perspective... If instead of g/mol, we referred to molecules/g -- we would end up populating tables and charts with really big numbers. This would make lookup tables hard to read, difficult to publish, and hard to work with. Imagine if you had to do math with a bunch of 10^23 exponents all of the time. Instead, it was agreed to effectively pull out a constant value from each of those to make the math significantly easier. Now, instead of dealing with a lot of big numbers, all of the lookup tables could now list smaller g/mol values. And we would be left with just the one single large (Avogadro's) number in the equations. Honestly, we don't need a set mole constant, but it makes chemistry significantly easier to do so. Unlike the other constants mentioned in the OP, Avogadro's number is completely arbitrary. It could be '1' as the parent suggested, except then it makes the rest of the math more difficult. Even for this SI overhaul, we didn't really even need to redefine the mole, except for the fact that it was previously defined in terms of the old kg. This was just "fixing a glitch".
- thomasahle 8y ago
  > Imagine if you had to do math with a bunch of 10^23 exponents all of the time. What if it was just set to 10^24 then? Much easier to remember and serves the same purpose. If it was 1 it would work too, since the SI system already has a way to deal with large numbers: prefixes. So we might wrote Ymol for yotta mole = 10^24 mol.
  - bonzini 8y ago
    Then 1 mol of carbon-12 would weigh 1300 g rather than 12 g and the conversions would be a pain.
- thaumasiotes 8y ago
  > Even for this SI overhaul, we didn't really even need to redefine the mole, except for the fact that it was previously defined in terms of the old kg. This was just "fixing a glitch". I came to complain about the article calling the mole a "base unit of the SI", and this seems like an appropriate thread. Why is the mole a defined unit at all? As far as I understand things, "one mole" is the same thing as Avogadro's number -- neither can be a unit, because they're both dimensionless constants (well, they're both one and the same dimensionless constant). Applying actual units, "one mole of water molecules" is the same thing as "Avogadro's number of water molecules". Avogadro's number, and therefore the mole, is the conversion factor between atomic mass units and grams. Similarly, 3 is the conversion factor between feet and yards, but nobody thinks 3 is a fundamental base unit of the imperial system. The foot is a base unit of the imperial system, measuring length, the yard is a non-base unit also measuring length, and 3 is a number with no special relationship to the system at all. It would be total nonsense to say that yards are defined by reference to 3. How is Avogadro's number different? Wouldn't "fixing the glitch" be abandoning the idea of calling the mole a unit in the first place?
  - int_19h 8y ago
    The new definition of mole is exactly that - it's just 6.02214076×10^23 items of whatever, and no longer depends on kilogram.
    - thaumasiotes 8y ago
      That's not a base unit, or a unit of any kind. What's it doing in a list of SI base units?
      - kiriakasis 8y ago
        It is a base unit, a unit for lots of stuff, and it is not dimensionless it is 6*10^23/mol
        thaumasiotes 8y ago
Aunche 8y ago
Why does it need to be based on the mass of an arbitrary molecule? Can't it just be 10^23 or 10^24? Most of the time, you're going to have to look up constants to convert moles to mass anyways.

filmor 8y ago

Carbon 12 is being used because that accounts for the nuclear forces which are absent for H-1.

mbreese 8y ago

I think this is exactly the rationale. But, from a more practical perspective... If instead of g/mol, we referred to molecules/g -- we would end up populating tables and charts with really big numbers. This would make lookup tables hard to read, difficult to publish, and hard to work with. Imagine if you had to do math with a bunch of 10^23 exponents all of the time. Instead, it was agreed to effectively pull out a constant value from each of those to make the math significantly easier. Now, instead of dealing with a lot of big numbers, all of the lookup tables could now list smaller g/mol values. And we would be left with just the one single large (Avogadro's) number in the equations. Honestly, we don't need a set mole constant, but it makes chemistry significantly easier to do so. Unlike the other constants mentioned in the OP, Avogadro's number is completely arbitrary. It could be '1' as the parent suggested, except then it makes the rest of the math more difficult. Even for this SI overhaul, we didn't really even need to redefine the mole, except for the fact that it was previously defined in terms of the old kg. This was just "fixing a glitch".

thomasahle 8y ago
> Imagine if you had to do math with a bunch of 10^23 exponents all of the time. What if it was just set to 10^24 then? Much easier to remember and serves the same purpose. If it was 1 it would work too, since the SI system already has a way to deal with large numbers: prefixes. So we might wrote Ymol for yotta mole = 10^24 mol.
- bonzini 8y ago
  Then 1 mol of carbon-12 would weigh 1300 g rather than 12 g and the conversions would be a pain.
thaumasiotes 8y ago
> Even for this SI overhaul, we didn't really even need to redefine the mole, except for the fact that it was previously defined in terms of the old kg. This was just "fixing a glitch". I came to complain about the article calling the mole a "base unit of the SI", and this seems like an appropriate thread. Why is the mole a defined unit at all? As far as I understand things, "one mole" is the same thing as Avogadro's number -- neither can be a unit, because they're both dimensionless constants (well, they're both one and the same dimensionless constant). Applying actual units, "one mole of water molecules" is the same thing as "Avogadro's number of water molecules". Avogadro's number, and therefore the mole, is the conversion factor between atomic mass units and grams. Similarly, 3 is the conversion factor between feet and yards, but nobody thinks 3 is a fundamental base unit of the imperial system. The foot is a base unit of the imperial system, measuring length, the yard is a non-base unit also measuring length, and 3 is a number with no special relationship to the system at all. It would be total nonsense to say that yards are defined by reference to 3. How is Avogadro's number different? Wouldn't "fixing the glitch" be abandoning the idea of calling the mole a unit in the first place?
- int_19h 8y ago
  The new definition of mole is exactly that - it's just 6.02214076×10^23 items of whatever, and no longer depends on kilogram.
  - thaumasiotes 8y ago
    That's not a base unit, or a unit of any kind. What's it doing in a list of SI base units?
    - kiriakasis 8y ago
      It is a base unit, a unit for lots of stuff, and it is not dimensionless it is 6*10^23/mol
      - thaumasiotes 8y ago

Aunche 8y ago

Why does it need to be based on the mass of an arbitrary molecule? Can't it just be 10^23 or 10^24? Most of the time, you're going to have to look up constants to convert moles to mass anyways.