The number of atoms in an amount of a substance

[g2c]

TL;DR Summary

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?

 

[mjc123]

Depends what "substance" you're considering. A molecule of hydrogen would comprise 1/N_A moles of H₂ molecules or 2/N_A moles of H atoms (but no free H atoms). If the "substance" is molecular hydrogen, the amount is 1/N_A moles. It has a molecular weight of 2 daltons, and a mass of 2/N_A g.

 

[Bandersnatch]

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.

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.

 

[jbriggs444]

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.

 

[g2c]

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/N_A since my sample has 1 particle (not one mole but one particle). How can it be??

 

[jbriggs444]

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/N_A 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.

 

[Mister T]

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/N_A 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.

 

[g2c]

@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?

 

[g2c]

I think I understood: The unite in the result of the division is the mol and so everything makes sens

 

[Mister T]

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!