The number of atoms in an amount of a substance

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Discussion Overview

The discussion revolves around the concept of the amount of substance, particularly in relation to hydrogen molecules and the Avogadro constant. Participants explore definitions, calculations, and interpretations of what constitutes a "discrete atomic-scale particle" in the context of moles and mass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants reference Wikipedia's definition of the amount of substance as the number of discrete atomic-scale particles divided by the Avogadro constant.
  • There is a distinction made between counting hydrogen atoms versus hydrogen molecules, with implications for the amount of substance calculated.
  • One participant asserts that a single H2 molecule should be considered as having an amount of substance of 1/NA, leading to questions about the interpretation of "discrete atomic-scale particle."
  • Another participant clarifies that if considering H2 as the discrete particle, then it would indeed have an amount of substance of 1/NA, but this raises further questions about definitions.
  • Some participants discuss the analogy of moles to other counting units, like a dozen, to clarify the concept.
  • There is a humorous comparison made regarding the mass of a mole of cows and moles of other entities, illustrating the scale of these quantities.
  • One participant mentions that the BIPM uses the term "elementary entity," suggesting a broader interpretation of what can be considered in these calculations.

Areas of Agreement / Disagreement

Participants express differing views on what constitutes a "discrete atomic-scale particle" and how to apply the concept of amount of substance to individual molecules versus moles. The discussion remains unresolved regarding the interpretation of these terms and their implications for calculations.

Contextual Notes

There are limitations in the assumptions made about the definitions of particles and entities, as well as the mathematical steps involved in the calculations. The discussion reflects varying interpretations and does not reach a consensus on these points.

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TL;DR
Help understand the definition of amount of substance
According to Wikipedia, its (quoting)
the number of discrete atomic-scale particles in it divided by the Avogadro constant

So a molecule of hydrogen would have an amount of substance of 2/NA and would have a mass of 2/(6*10^23) daltons?
 
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Depends what "substance" you're considering. A molecule of hydrogen would comprise 1/NA moles of H2 molecules or 2/NA moles of H atoms (but no free H atoms). If the "substance" is molecular hydrogen, the amount is 1/NA moles. It has a molecular weight of 2 daltons, and a mass of 2/NA g.
 
g2c said:
So a molecule of hydrogen would have an amount of substance of 2/NA
Yes, but only if you're counting hydrogen atoms. Which is then 2/(6*10^23) moles of H. (using rounded values here)
If you're counting hydrogen molecules, then the single discrete particle is that molecule, and the amount of 1 molecule of H2 is 1/(6*10^23) moles of H2.
g2c said:
and would have a mass of 2/(6*10^23) daltons?
No. It would be 2 daltons. (again, all values rounded)

The concept of moles can be confusing at first. But it helps to realize it's just a number, no different than something like e.g. 'a dozen'.
A dozen is 12 of whatever you're counting.
So e.g. 2 cows are 2/12 of a dozen cows.
2 hydrogen atoms are 2/12 of a dozen hydrogen atoms.
1 H2 molecule is 1/12 of a dozen H2 molecules.

A mole is 6*10^23 of whatever you're counting.
So e.g. 2 cows are 2/6*10^23 of a mole of cows.
2 hydrogen atoms are 2/6*10^23 of a mole of hydrogen atoms.
1 H2 molecule is 1/6*10^23 of a mole of H2 molecules.

A dalton is 1/12 of the mass of a C-12 atom.
Since a C-12 atom has mass of 12 daltons, and a mole (so, 6*10^23) of C-12 atoms has a mass of ``12 grams, it follows that if you have a mole of a different substance, and a single unit of that substance weighs N daltons, then a mole of that substance will weigh N grams.

So e.g. if 1 hydrogen atom has the mass of 1 dalton, a mole of hydrogen atoms weighs 1 gram.
If 1 hydrogen molecule has the mass of 2 daltons, a mole of hydrogen molecules weighs 2 grams.Incidentally, a large cow has a mass of approx. 1 metric ton (10^6 g). A mole of cows would weigh (10^6)*(6*10^23) = 6*10^29 grams. So a single cow's mass in daltons is 6*10^29 daltons.
 
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Bandersnatch said:
So a single cow's mass in daltons is 6*10^29 daltons.
Meanwhile, a mole (burrowing animal) weighs about 100 grams. So a mole of moles is 6 * 10^23 * 6 * 10^23 * 100 = 3.6 * 10^49 daltons.

With apologies to xkcd.
 
My substance is 1 H2 molecule. According to Wikipedia "the number of discrete atomic-scale particles in it divided by the Avogadro constant" it would have an amount of substance of 1/NA since my sample has 1 particle (not one mole but one particle). How can it be??
 
g2c said:
My substance is 1 H2 molecule. According to Wikipedia "the number of discrete atomic-scale particles in it divided by the Avogadro constant" it would have an amount of substance of 1/NA since my sample has 1 particle (not one mole but one particle). How can it be??
Your sample has ##\frac{1}{6.02 \times 10^{23}}## moles of H2 molecules.
 
g2c said:
My substance is 1 H2 molecule. According to Wikipedia "the number of discrete atomic-scale particles in it divided by the Avogadro constant" it would have an amount of substance of 1/NA since my sample has 1 particle (not one mole but one particle). How can it be??

Because you're considering the H2 molecule to be what Wikipedia is calling a "discrete atomic-scale particle".

By the way, BIPM uses the term "elementary entity", not "discrete atomic-scale particle". BIPM is the authority. So in your case the elementary entity is a H2 molecule.
 
@Mister T
yes, I consider one H2 molecule as elementary particle and want to know what's its amount of substance according to the definition. What's wrong?? and if it is wrong, what is it , according to you, that is allowed to be taken as elementary particle?
 
I think I understood: The unite in the result of the division is the mol and so everything makes sens
 
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g2c said:
according to you, that is allowed to be taken as elementary particle?

Elementary entity. It could be a H2 molecule. It could be an egg!
 

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