8 ms·

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fund

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

deleted 8y ago
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AstralStorm 8y ago
For the same reason count is not a mass. Avogadro constant is a count relating number of specified atoms to mass.
- 3pt14159 8y ago
  Isn't this something essentially tautological though? When we discuss the mass of a neutron and we say "one neutron weighs one u" then we discuss the mass of an electron and we say "one electron is 5.4858×10−4 u" and "one proton is 1.0072764 u" then we add them up and say "one hydrogen atom is 1.00794 u while one helium atom is 4.002602" (forgetting some complications for a moment) are we not just summing likes? Or is it just that since mass is defined in Planck and time / distance terms that we need to relate it to counts of things? Theres a gap there I don't understand. Can we not just say "we measured a proton's mass and it is u"? Am I making a jump there?
jjjensen90 8y ago
In my opinion it is because a mole is a numerical amount of atoms vs the mass of the substance, so it makes reactions, formulas, and measurements easier to determine. Avogadro's number is useful just as a baseline to use (number of carbon-12 atoms in 12 grams of carbon-12), like there are 12 inches in a foot or 100 centimeters in a meter...
- out_of_protocol 8y ago
  ... except if you take 6 two-inches you'll see something different than 12 one-inches. Our universe is weird. Defect of mass and stuff
DoofusOfDeath 8y ago
In a meta-sense, I don't think your question is dumb at all. There's a complicated technical topic which you're still not understanding. There's no indication it's a question you could easily answer yourself, and you're posting it in a forum of people likely to find the topic interesting, some of whom might give an answer that clicks for you. Well done, IMHO.
Zarel 8y ago
It's not a fundamental constant. > the Avogadro number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydrogen, meaning one gram of hydrogen. (from Wikipedia) (It's since been refined to be 12 grams of carbon-12.) So a mole is defined to be approximately one gram worth of protons and neutrons. We use it because grams are a significantly easier unit of mass for humans to work with, than like individual particles.
- garmaine 8y ago
  Read the OP. It is now a fundamental constant. (But you’re right if you’re referring to the status quo before).
  - Zarel 8y ago
    No, it's not. Avogadro's number is a constant, but it's not fundamental - the only difference is that it's defined in terms of fundamental constants (namely, the Planck constant) now. It's still an arbitrary number defined in terms of the mass of a gram.
    - garmaine 8y ago
      Nope, it is fixed to be N_A = 602,214,076,000,000,000,000,000 exactly (6.02214076x10^23 to units precision). It used to be the case that the mole was an experimental value equal to the number of atoms in a certain mass of a certain something. That is no longer the case with this revision. It is a fixed, never changing integer constant. This does mean that 1 mole of carbon-12 is no longer exactly 12 grams. But it is approximately 12.0000000 grams, which is within the best we can experimentally measure today, so nothing changes in practice as a result of this update except first chapter of an introductory chemistry textbook (good excuse to push out a 9th edition for $250!). Therefore it is accurate to say now that whereas before the Avogadro's number was experimentally determined based the exact expressed mass of a carbon-12 atom relative to a platinum-iridium cylinder in Paris (the old kg), it is now the case that the expressed mass of the carbon-12 atom must be measured relative to the a kg definition based on Planck's constant. (I say "expressed mass" because this situation is a little confusing... I'm talking about the numbers we write down expressing the mass as a multiple of some standard kilogram. That reference mass changed, not the actual inertial mass.) EDIT: Or you can just read the draft of the agreement that was voted on. The definitions are on the first page: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN...
FPGAhacker 8y ago
I thought it was just a ratio. Mass per mole. I admit I have never used this in my day to day life and college was 20 years ago. So maybe I am clueless.
- tetha 8y ago
  Nah, I think that's just it. It's easier to write down certain calculations in mole in a chemical context, because you're concerned with atoms reacting with each other. You don't have 12 grams of carbon and 4 grams of hydrogen, but 1 mole of carbon and 4 mole of hydrogen.
adamnemecek 8y ago
If it makes you feel any better, I've asked the same thing before.
leni536 8y ago
My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly. Edit: I think your question boils down to "Why do we have two separate units for mass, u and kg, connected by the Avogadro constant?" Most answers dismiss your original question as Avogadro constant is not a unit. But u is a unit and it's the point why we have this constant. Edit2: To further emphasize my point look at the mass of neutron[1]. It's listed both in kg and u. Note the number of decimal places. [1] https://en.wikipedia.org/wiki/Neutron https://en.wikipedia.org/wiki/Neutron
- 3pt14159 8y ago
  Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass. Why do I need both kg and u as fundamental constants? I believe the comments here have answered it. It isn't something weird, like quantum gravity or some such. If I understand everyone correctly it's just a practical decision we made at some point because we didn't want mass to be in u and that's that. I feel better about it now.
  - jessriedel 8y ago
    They aren't fundamental constants, they are fundamental units. The system is ultimately about allowing people to compare measurements (not describing the universe in abstract), and is only tied to fundamental physical constants where it makes that more reliable. That's not a whole answer, but it may be helpful for your thinking.
XorNot 8y ago
As others have noted, knowing the count of entities (note: not atoms, entities - i.e. it can also be molecules) in relation to actual mass is very useful for the physical sciences - at a molecular level the quantities of molecules and atoms interacting matter, not their masses - but mass is the unit I do experiments with. EDIT: A simple example would be if you were trying to make water - H2O, from hydrogen (H2) and oxygen (O2). The molar ratio is 2:1 - but in doing a practical synthesis, that doesn't tell me how much mass/volume of gases to actually mix up. Avogadro's number and molar mass is what I need to turn those into practical units to work with.
- aylmao 8y ago
  I think what the commenter is referring to is why use Avogadro's number instead of say, just Sextillions or Septillions.
  - XorNot 8y ago
    I mean that really comes back to the "practical units" thing (and that chemists were the ones doing the work) - a kilogram of anything in chemistry is a huge amount, whereas a gram is about right to make "a lot" of something (in synthesis research people get excited about gram-scale synthetic yields). So it's pretty much chosen to get integer-ish units with common things you work with like carbon - i.e. 1 mole of carbon of is ~12 grams.
    - aylmao 8y ago
      Yeah, that makes sense (:
pfortuny 8y ago
Simplistically: two moles of protons+two moles of neutrons do not weight the same as a mole of nuclei of He-4, as far as I know.
Jedi72 8y ago
No I agree, it seems stupid. What we did is take a block of metal and call it a kilogram, figure out how many atoms are in 1/1000 of it (a gram) and call that a mole. What we should do is redefine the gram to be whatever 1e10 atoms of Hydrogen weighs, or something similar. When I read the headline I thought thats what they did, and I admit first reaction was "oh god I'm going to be dealing with conversions and associated errors for the rest of my life". Disclaimer: I took high school chem, that's it.
- pbhjpbhj 8y ago
  1E10 atoms of hydrogen isn't a fixed mass. Cf binding energy, electron excitation, E=mc^2 (the m there is m0, the rest mass; the full equation includes the momentum and "relativistic mass"). The situation is actually worse for hydrogen (cf hydrogen bonding). http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.htm... is a pretty coherent and readable approach to the subject.
- bonzini 8y ago
  > What we should do is redefine the gram to be whatever 1e10 atoms of Hydrogen weighs, or something similar. That was actually an alternative proposal for redefining the kg. The kg would have been 1000/28 the weight of a mole of silicon-28; you could build a sample by counting 6.023x10^26/28 atoms of silicon-28, and making a sphere out of them. Initially the watt balance seemed to be less precise than atom counting, but then it was improved to a point where defining the kg on top of the mole became less convenient than the definition they are adopting now.
weinzierl 8y ago
leni536’s answer is the real answer but I’ll give you another more meta one: Because the SI system is redundant anyway one constant more or less doesn’t matter much. If you want a truly minimalist system you can use CGM or MKS.
Koshkin 8y ago
In chemistry, the molar “mass” plays a more important role than the mass proper. Why has its unit not been chosen to be equal to 1? For the same reason as why the gram is not defined to be the mass of, say, proton. (Chemistry doesn’t normally deal with single molecules.)
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namirez 8y ago
For the same reason that we use both temperature and energy and find them both useful. We can talk of the energy of an electron. On the other hand, temperature, is an statistical property defined for many particles. Using mole (based on Avogadro constant) makes it easier to do statistical mechanics, but it's not a microscopic property like mass.
jcranmer 8y ago
Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspects of chemistry end up being pretty related to molecule counts. Vanilla mass doesn't cut it since an atom of iodine weighs about 6.6 times that of fluorine but can still only react with one other molecule. The flip side is that the molecular count is less useful to us in the everyday world. We can gauge the weight of a kilogram much more than we can gauge a septillion molecules. And if we're trying to figure out how much stuff a shelf can hold before it collapses, it's the weight that matters, not the actual molecular count. (Note for pedants: in the familiar environment of Earth's surface, mass and weight can be treated as the same quantity in most cases.) So mass and molecular count are both very important quantities that have importance in different fields of science, and they don't have a trivial relationship to each other. Avogadro's constant and molar mass is a way to express their relationship.
- zwkrt 8y ago
  trying to use kg instead of moles when calculating a chemical reaction would be like trying to set up a speed dating event by taking the weights of all the men and women instead of matching them up pairwise. Sometimes it is much easier to calculate based on scalar quantity than it is to calculate based on mass.
  - dfox 8y ago
    The argument there is that avogadro's constant is just an arbitrary number and mol is not an unit, but weird case of SI prefix. On the other hand it is fundamental-ish constant, because it is defined as arbitrarily scaled result of inherently uncertain measurement. And the new definition of kilogram had significantly increased the attainable certainity of such measurement (to the extent that it is uncertain due to practical issues, not by definition). The other proposed replicable definition of kilogram (ie. mass of Si monocrystal wih particular geometry) would fix the definition of mole as some known and defined number, but would be significantly harder to replicate (because it would define how to produce an artifact in contrast to how to directly measure the mass as the ratified definition does)
da_chicken 8y ago
The one that made it click for me as incredibly useful was Avogrado's Law (now part of the Ideal Gas Law): > Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. This law, for example, explains why hot air rises. Take two equal quanties of the same gas at the same pressure. They will have the same volume. Now, heat one quantity of gas. By this law, that gas will have a larger volume at the same pressure. Because it has the same amount of mass distributed over a larger volume, it must be less dense. Therefore, it rises.
- tantalor 8y ago
  > By this law, that gas will have a larger volume The law as quoted doesn't say what happens when the temperature change. Maybe it gets more dense? Be more specific.
- effie 8y ago
  > By this law, that gas will have a larger volume at the same pressure. It will have larger volume, but that is not implied by Avogadro's law. That law is about the surprising property of all gases: no matter the chemical nature of the gases, if they all have same T and P, they all will have same number of molecules per unit volume.
jimktrains2 8y ago
Stoichiometry. We need a means of converting mass to number of atoms so that we can predict how much mass of a specific product will be formed, what the limiting reägent will be, &c. It also helps to define concentrations based on number of moles in a L of solvent (Molarity vs g/L) for the same reason.
aaronblohowiak 8y ago
The candela is a real abomination of a unit.
- ben509 8y ago
  Absolutely, it's candelous that it's part of the standard.
madhadron 8y ago
Historically, the mole predates the acceptance of atomic theory. Stoichiometry of various reactions let you work out that there was some mass of oxygen that would entirely react with some mass of carbon to form carbon monoxide. They didn't have the same masses, but the relative amounts for each element were constant (or small integer multiples, such as twice as much oxygen to carbon for CO_2). So Dalton took the lightest one, hydrogen, and defined a mole as the stoichiometric amount equivalent to that in 1g of hydrogen. Looked at that way, it's a pretty solid choice.
garmaine 8y ago
Well, now you could do that. At least you could next March when these rules take affect. But before you could not because the conversion between atomic mass and SI/kg mass depended on that experimental constant. Two mass systems were required because we couldn’t conver between them with atomic accuracy.
amptorn 8y ago
I don't believe the Avogadro constant needs to be an SI unit. It's like having 12 as an SI unit, or a million, or 1. Sure, it's a useful constant scaling factor, but it doesn't need to be canonized at the heart of the SI system. It doesn't actually express anything fundamental about how we measure our universe. And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done.
- thomasahle 8y ago
  > And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done. No longer is. Not quite.
j1vms 8y ago
> why we even needed the Avogadro constant / mole as a fundamental constant(...?) It serves as a link between human-scale and atomic-scale observations, classically needed for chemistry performed on Earth by humans. This link must exist, as others mentioned, for stoichiometric calculations (e.g. air/gas mixture in a internal combustion engine). For much of modern scientific history, it was deemed useful to scale into an easily eye-visible human scale (the gram). [0] [0] https://en.wikipedia.org/wiki/Avogadro_constant#General_role_in_science https://en.wikipedia.org/wiki/Avogadro_constant#General_role...
grogers 8y ago
The number of things in a mole is arbitrary. It's a dimensionless unit. However, since many things are already measured in moles in chemistry, there's no real reason to remove it. Dealing with numbers on a more practical macroscopic scale is probably more convenient than dealing with large powers of 10. https://en.wikipedia.org/wiki/Mole_(unit)#Criticism https://en.wikipedia.org/wiki/Mole_(unit)#Criticism

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AstralStorm 8y ago

For the same reason count is not a mass. Avogadro constant is a count relating number of specified atoms to mass.

3pt14159 8y ago
Isn't this something essentially tautological though? When we discuss the mass of a neutron and we say "one neutron weighs one u" then we discuss the mass of an electron and we say "one electron is 5.4858×10−4 u" and "one proton is 1.0072764 u" then we add them up and say "one hydrogen atom is 1.00794 u while one helium atom is 4.002602" (forgetting some complications for a moment) are we not just summing likes? Or is it just that since mass is defined in Planck and time / distance terms that we need to relate it to counts of things? Theres a gap there I don't understand. Can we not just say "we measured a proton's mass and it is u"? Am I making a jump there?

jjjensen90 8y ago

In my opinion it is because a mole is a numerical amount of atoms vs the mass of the substance, so it makes reactions, formulas, and measurements easier to determine. Avogadro's number is useful just as a baseline to use (number of carbon-12 atoms in 12 grams of carbon-12), like there are 12 inches in a foot or 100 centimeters in a meter...

out_of_protocol 8y ago
... except if you take 6 two-inches you'll see something different than 12 one-inches. Our universe is weird. Defect of mass and stuff

DoofusOfDeath 8y ago

In a meta-sense, I don't think your question is dumb at all. There's a complicated technical topic which you're still not understanding. There's no indication it's a question you could easily answer yourself, and you're posting it in a forum of people likely to find the topic interesting, some of whom might give an answer that clicks for you. Well done, IMHO.

Zarel 8y ago

It's not a fundamental constant. > the Avogadro number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydrogen, meaning one gram of hydrogen. (from Wikipedia) (It's since been refined to be 12 grams of carbon-12.) So a mole is defined to be approximately one gram worth of protons and neutrons. We use it because grams are a significantly easier unit of mass for humans to work with, than like individual particles.

garmaine 8y ago
Read the OP. It is now a fundamental constant. (But you’re right if you’re referring to the status quo before).
- Zarel 8y ago
  No, it's not. Avogadro's number is a constant, but it's not fundamental - the only difference is that it's defined in terms of fundamental constants (namely, the Planck constant) now. It's still an arbitrary number defined in terms of the mass of a gram.
  - garmaine 8y ago
    Nope, it is fixed to be N_A = 602,214,076,000,000,000,000,000 exactly (6.02214076x10^23 to units precision). It used to be the case that the mole was an experimental value equal to the number of atoms in a certain mass of a certain something. That is no longer the case with this revision. It is a fixed, never changing integer constant. This does mean that 1 mole of carbon-12 is no longer exactly 12 grams. But it is approximately 12.0000000 grams, which is within the best we can experimentally measure today, so nothing changes in practice as a result of this update except first chapter of an introductory chemistry textbook (good excuse to push out a 9th edition for $250!). Therefore it is accurate to say now that whereas before the Avogadro's number was experimentally determined based the exact expressed mass of a carbon-12 atom relative to a platinum-iridium cylinder in Paris (the old kg), it is now the case that the expressed mass of the carbon-12 atom must be measured relative to the a kg definition based on Planck's constant. (I say "expressed mass" because this situation is a little confusing... I'm talking about the numbers we write down expressing the mass as a multiple of some standard kilogram. That reference mass changed, not the actual inertial mass.) EDIT: Or you can just read the draft of the agreement that was voted on. The definitions are on the first page: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN...

FPGAhacker 8y ago

I thought it was just a ratio. Mass per mole. I admit I have never used this in my day to day life and college was 20 years ago. So maybe I am clueless.

tetha 8y ago
Nah, I think that's just it. It's easier to write down certain calculations in mole in a chemical context, because you're concerned with atoms reacting with each other. You don't have 12 grams of carbon and 4 grams of hydrogen, but 1 mole of carbon and 4 mole of hydrogen.

adamnemecek 8y ago

If it makes you feel any better, I've asked the same thing before.

leni536 8y ago

My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly. Edit: I think your question boils down to "Why do we have two separate units for mass, u and kg, connected by the Avogadro constant?" Most answers dismiss your original question as Avogadro constant is not a unit. But u is a unit and it's the point why we have this constant. Edit2: To further emphasize my point look at the mass of neutron[1]. It's listed both in kg and u. Note the number of decimal places. [1] https://en.wikipedia.org/wiki/Neutron https://en.wikipedia.org/wiki/Neutron

3pt14159 8y ago
Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass. Why do I need both kg and u as fundamental constants? I believe the comments here have answered it. It isn't something weird, like quantum gravity or some such. If I understand everyone correctly it's just a practical decision we made at some point because we didn't want mass to be in u and that's that. I feel better about it now.
- jessriedel 8y ago
  They aren't fundamental constants, they are fundamental units. The system is ultimately about allowing people to compare measurements (not describing the universe in abstract), and is only tied to fundamental physical constants where it makes that more reliable. That's not a whole answer, but it may be helpful for your thinking.

XorNot 8y ago

As others have noted, knowing the count of entities (note: not atoms, entities - i.e. it can also be molecules) in relation to actual mass is very useful for the physical sciences - at a molecular level the quantities of molecules and atoms interacting matter, not their masses - but mass is the unit I do experiments with. EDIT: A simple example would be if you were trying to make water - H2O, from hydrogen (H2) and oxygen (O2). The molar ratio is 2:1 - but in doing a practical synthesis, that doesn't tell me how much mass/volume of gases to actually mix up. Avogadro's number and molar mass is what I need to turn those into practical units to work with.

aylmao 8y ago
I think what the commenter is referring to is why use Avogadro's number instead of say, just Sextillions or Septillions.
- XorNot 8y ago
  I mean that really comes back to the "practical units" thing (and that chemists were the ones doing the work) - a kilogram of anything in chemistry is a huge amount, whereas a gram is about right to make "a lot" of something (in synthesis research people get excited about gram-scale synthetic yields). So it's pretty much chosen to get integer-ish units with common things you work with like carbon - i.e. 1 mole of carbon of is ~12 grams.
  - aylmao 8y ago
    Yeah, that makes sense (:

pfortuny 8y ago

Simplistically: two moles of protons+two moles of neutrons do not weight the same as a mole of nuclei of He-4, as far as I know.

Jedi72 8y ago

No I agree, it seems stupid. What we did is take a block of metal and call it a kilogram, figure out how many atoms are in 1/1000 of it (a gram) and call that a mole. What we should do is redefine the gram to be whatever 1e10 atoms of Hydrogen weighs, or something similar. When I read the headline I thought thats what they did, and I admit first reaction was "oh god I'm going to be dealing with conversions and associated errors for the rest of my life". Disclaimer: I took high school chem, that's it.

pbhjpbhj 8y ago
1E10 atoms of hydrogen isn't a fixed mass. Cf binding energy, electron excitation, E=mc^2 (the m there is m0, the rest mass; the full equation includes the momentum and "relativistic mass"). The situation is actually worse for hydrogen (cf hydrogen bonding). http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.htm... is a pretty coherent and readable approach to the subject.
bonzini 8y ago
> What we should do is redefine the gram to be whatever 1e10 atoms of Hydrogen weighs, or something similar. That was actually an alternative proposal for redefining the kg. The kg would have been 1000/28 the weight of a mole of silicon-28; you could build a sample by counting 6.023x10^26/28 atoms of silicon-28, and making a sphere out of them. Initially the watt balance seemed to be less precise than atom counting, but then it was improved to a point where defining the kg on top of the mole became less convenient than the definition they are adopting now.

weinzierl 8y ago

leni536’s answer is the real answer but I’ll give you another more meta one: Because the SI system is redundant anyway one constant more or less doesn’t matter much. If you want a truly minimalist system you can use CGM or MKS.

Koshkin 8y ago

In chemistry, the molar “mass” plays a more important role than the mass proper. Why has its unit not been chosen to be equal to 1? For the same reason as why the gram is not defined to be the mass of, say, proton. (Chemistry doesn’t normally deal with single molecules.)

deleted 8y ago

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namirez 8y ago

For the same reason that we use both temperature and energy and find them both useful. We can talk of the energy of an electron. On the other hand, temperature, is an statistical property defined for many particles. Using mole (based on Avogadro constant) makes it easier to do statistical mechanics, but it's not a microscopic property like mass.

jcranmer 8y ago

Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspects of chemistry end up being pretty related to molecule counts. Vanilla mass doesn't cut it since an atom of iodine weighs about 6.6 times that of fluorine but can still only react with one other molecule. The flip side is that the molecular count is less useful to us in the everyday world. We can gauge the weight of a kilogram much more than we can gauge a septillion molecules. And if we're trying to figure out how much stuff a shelf can hold before it collapses, it's the weight that matters, not the actual molecular count. (Note for pedants: in the familiar environment of Earth's surface, mass and weight can be treated as the same quantity in most cases.) So mass and molecular count are both very important quantities that have importance in different fields of science, and they don't have a trivial relationship to each other. Avogadro's constant and molar mass is a way to express their relationship.

zwkrt 8y ago
trying to use kg instead of moles when calculating a chemical reaction would be like trying to set up a speed dating event by taking the weights of all the men and women instead of matching them up pairwise. Sometimes it is much easier to calculate based on scalar quantity than it is to calculate based on mass.
- dfox 8y ago
  The argument there is that avogadro's constant is just an arbitrary number and mol is not an unit, but weird case of SI prefix. On the other hand it is fundamental-ish constant, because it is defined as arbitrarily scaled result of inherently uncertain measurement. And the new definition of kilogram had significantly increased the attainable certainity of such measurement (to the extent that it is uncertain due to practical issues, not by definition). The other proposed replicable definition of kilogram (ie. mass of Si monocrystal wih particular geometry) would fix the definition of mole as some known and defined number, but would be significantly harder to replicate (because it would define how to produce an artifact in contrast to how to directly measure the mass as the ratified definition does)

da_chicken 8y ago

The one that made it click for me as incredibly useful was Avogrado's Law (now part of the Ideal Gas Law): > Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. This law, for example, explains why hot air rises. Take two equal quanties of the same gas at the same pressure. They will have the same volume. Now, heat one quantity of gas. By this law, that gas will have a larger volume at the same pressure. Because it has the same amount of mass distributed over a larger volume, it must be less dense. Therefore, it rises.

tantalor 8y ago
> By this law, that gas will have a larger volume The law as quoted doesn't say what happens when the temperature change. Maybe it gets more dense? Be more specific.
effie 8y ago
> By this law, that gas will have a larger volume at the same pressure. It will have larger volume, but that is not implied by Avogadro's law. That law is about the surprising property of all gases: no matter the chemical nature of the gases, if they all have same T and P, they all will have same number of molecules per unit volume.

jimktrains2 8y ago

Stoichiometry. We need a means of converting mass to number of atoms so that we can predict how much mass of a specific product will be formed, what the limiting reägent will be, &c. It also helps to define concentrations based on number of moles in a L of solvent (Molarity vs g/L) for the same reason.

aaronblohowiak 8y ago

The candela is a real abomination of a unit.

ben509 8y ago
Absolutely, it's candelous that it's part of the standard.

madhadron 8y ago

Historically, the mole predates the acceptance of atomic theory. Stoichiometry of various reactions let you work out that there was some mass of oxygen that would entirely react with some mass of carbon to form carbon monoxide. They didn't have the same masses, but the relative amounts for each element were constant (or small integer multiples, such as twice as much oxygen to carbon for CO_2). So Dalton took the lightest one, hydrogen, and defined a mole as the stoichiometric amount equivalent to that in 1g of hydrogen. Looked at that way, it's a pretty solid choice.

garmaine 8y ago

Well, now you could do that. At least you could next March when these rules take affect. But before you could not because the conversion between atomic mass and SI/kg mass depended on that experimental constant. Two mass systems were required because we couldn’t conver between them with atomic accuracy.

amptorn 8y ago

I don't believe the Avogadro constant needs to be an SI unit. It's like having 12 as an SI unit, or a million, or 1. Sure, it's a useful constant scaling factor, but it doesn't need to be canonized at the heart of the SI system. It doesn't actually express anything fundamental about how we measure our universe. And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done.

thomasahle 8y ago
> And if nothing else, it can be derived: a mole is the number of atoms in a kilogram of carbon-12. Done. No longer is. Not quite.

j1vms 8y ago

> why we even needed the Avogadro constant / mole as a fundamental constant(...?) It serves as a link between human-scale and atomic-scale observations, classically needed for chemistry performed on Earth by humans. This link must exist, as others mentioned, for stoichiometric calculations (e.g. air/gas mixture in a internal combustion engine). For much of modern scientific history, it was deemed useful to scale into an easily eye-visible human scale (the gram). [0] [0] https://en.wikipedia.org/wiki/Avogadro_constant#General_role_in_science https://en.wikipedia.org/wiki/Avogadro_constant#General_role...

grogers 8y ago

The number of things in a mole is arbitrary. It's a dimensionless unit. However, since many things are already measured in moles in chemistry, there's no real reason to remove it. Dealing with numbers on a more practical macroscopic scale is probably more convenient than dealing with large powers of 10. https://en.wikipedia.org/wiki/Mole_(unit)#Criticism https://en.wikipedia.org/wiki/Mole_(unit)#Criticism