# The number of atoms in an amount of a substance

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?

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mjc123
Homework Helper
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.

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 realise 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.

DrClaude
jbriggs444
Homework Helper
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??

jbriggs444
Homework Helper
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.

Mister T
Gold Member
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 whats 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

Mister T