by yhPscis
Tags: clarifications, mole
P: 17
 Quote by Borek It is not mass of a nucleon that is defined, it is a UNIT used to measure it (amu) that is defined. It is defined to be 1/12 of the mass of the C-12 atom. Close to the mass of the nucleon, but different.
But if 1 amu does not equal the mass of a nucleon, then the mass of the nucleon can't be canceled out in the equaltion:

1 amu = 1 g/NA

NA = 1g/1 amu

Mass of atom = number of nucleons x mass of 1 nucleon

NA x Mass of atom = NA x number of nucleons x mass of 1 nucleon

1g/1 amu x mass of atom = 1g/1 amu x number of nucleons x mass of 1 nucleon

You can't say:

Mass of nucleon/amu x number of nucleons = 1g x number of nucleons

So

NA x mass of atom = number of nucleons g

So how come having an Avogadro number (NA) of atoms of an element equals the number of nucleons of that element in grams anyway?

Why didn't they use the reciprocal of the mass of a nucleon instead of Avogadro's number although the former fits the equation perfectly (and the latter does not)?
 HW Helper P: 1,965 Reflect, all measurements are comparisons. It is essential to have some standard to compare that back to, and since the nineteenth century the standards have been internationalised. And it is good to make them as convenient as reasonably possible. When you ask in #what a coincidence! how come it worked out that simple it's like saying hey a litre of water turns out to weigh exactly one kilo, incredible coincidence!
P: 17
 Quote by epenguin Reflect, all measurements are comparisons. It is essential to have some standard to compare that back to, and since the nineteenth century the standards have been internationalised. And it is good to make them as convenient as reasonably possible. When you ask in #what a coincidence! how come it worked out that simple it's like saying hey a litre of water turns out to weigh exactly one kilo, incredible coincidence!
Oh, I understand that people look for the most convenient way to measure things. Before it was redefined as 1/1000 of the mass of the international prototype of the kilogram in France, the gram was defined as the mass of one cubic centimeter of water (according to wiki), so I guess it makes sense that one liter of water turns out to have a mass of 1 kilogram. It's a logical extension of that arbitrary definition of the gram.

But it doesn't seem to make sense that if you take NA amount of atoms of a substance, you turn out to have the number of nucleons in grams of that substance because, as shown in my first post on this page, the equations don't fit... the logic behind that practice seems erroneous...

How come this logic is used despite that the equations don't fit? Why don't they use the mass of a nucleon instead of Avogadro's number?
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P: 6,334
 Quote by yhPscis For example, carbon has an atomic mass number of 12 so 1 mole of carbon equals 12 grams of carbon.
That's the definition. Before 1961, oxygen was defined as having 1 mole O = 16 grams, but now
1 mole O = 15.999 grams. Is the oxygen from before 1961 different from post-1961 oxygen? No, it isn't. We've just changed the weight slightly.

 Hydrogen has an atomic mass number of 1 so 1 mole of carbon hydrogen equals 1 gram of hydrogen.
This is not entirely correct. 1 mole of H-1 = 1.007 825 0 grams, due to 1 mole of C-12 = 12 grams exactly, by definition.

 Is this some great coincidence? Or is there a link that I'm not seeing? I'm not sure I understood what a mole is and my research on it just confuses me, so there's why I'm asking. Thank you for reading and (hopefully) helping!
No. As pointed out, it is by design.

http://en.wikipedia.org/wiki/Atomic_mass_unit

P: 23,397
 Quote by yhPscis So how come having an Avogadro number (NA) of atoms of an element equals the number of nucleons of that element in grams anyway?
In general - it doesn't. It does only approximately. Have you read my earlier post about mass of the helium nucleus? Not only mass of the nucleus doesn't equal sum of the masses of the nucleons involved, but the difference is different for every isotope.

 Why didn't they use the reciprocal of the mass of a nucleon instead of Avogadro's number although the former fits the equation perfectly (and the latter does not)?
C-12 was an arbitrary choice, as was the earlier choice of O-16. I don't remember exact reasoning behind, for sure it can be googled - but the choice was not made by a bunch of monkeys, so I trust their reason.

Besides - which nucleon? Proton, or neutron? The difference is around /1830, so quite large.
P: 17
 Quote by Borek In general - it doesn't. It does only approximately. Have you read my earlier post about mass of the helium nucleus? Not only mass of the nucleus doesn't equal sum of the masses of the nucleons involved, but the difference is different for every isotope. C-12 was an arbitrary choice, as was the earlier choice of O-16. I don't remember exact reasoning behind, for sure it can be googled - but the choice was not made by a bunch of monkeys, so I trust their reason. Besides - which nucleon? Proton, or neutron? The difference is around /1830, so quite large.
Oh, I see, I mistakenly thought it didn't matter which nucleon because the difference of mass between both nucleons was negligible.

So, If I understood it well, by using Avogadro's number, you only get an approximation of the mass, but they use that method anyway because, although it's not completely accurate, it's accurate enough to achieve the chemical reactions you want in a lab.