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I need some clarifications about the mole, please 
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#19
Apr1114, 07:51 AM

P: 17

1 amu = 1 g/N_{A} N_{A} = 1g/1 amu Mass of atom = number of nucleons x mass of 1 nucleon N_{A} x Mass of atom = N_{A} 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 N_{A} x mass of atom = number of nucleons g So how come having an Avogadro number (N_{A}) 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)? 


#20
Apr1114, 11:30 AM

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! 


#21
Apr1114, 12:06 PM

P: 17

But it doesn't seem to make sense that if you take N_{A} 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? 


#22
Apr1114, 01:41 PM

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Thanks
PF Gold
P: 6,334

1 mole O = 15.999 grams. Is the oxygen from before 1961 different from post1961 oxygen? No, it isn't. We've just changed the weight slightly. http://en.wikipedia.org/wiki/Atomic_mass_unit http://en.wikipedia.org/wiki/Avogadro_constant 


#23
Apr1114, 02:25 PM

Admin
P: 23,397

Besides  which nucleon? Proton, or neutron? The difference is around /1830, so quite large. 


#24
Apr1114, 07:26 PM

P: 17

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. Thanks for your explanations (and your patience) 


#25
Apr1214, 03:08 AM

Admin
P: 23,397

Please remember we measure atomic mass. We don't calculate atomic mass by summing masses of nucleons, quite the opposite, we deduct their number from the measured atomic mass and from the spectroscopically determined nucleon charge. 


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