# 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?

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## Answers and Replies

Science Advisor
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.

Science Advisor
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.

DrClaude
Science Advisor
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.

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

Science Advisor
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.

Science Advisor
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.

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

Science Advisor
Gold Member
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!