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Why is 1 u = 1.6605 x 10^-27 kg?

  1. Jul 23, 2012 #1
    Sorry if this is a dumb question.. maybe I'm largely overlooking something..

    For example, 1 mol of N2 (diatomic nitrogen gas) is apparently equal to 2 x (14u) x 1.6605 x 10^-27 kg/u = 4.6494 x 10^-26 kg

    Why is 1 mol of N2 not equal to (1 mol N2) x 28 g N2/1 mol N2 = 28 g = .028 kg??

    These two values are quite different..

    Again, sorry it this is a dumb question!! I think there's something I'm not understanding??
  2. jcsd
  3. Jul 23, 2012 #2


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    You are mis-reading something, or there is a misprint. 1 mole of atoms weighs the atomic weight in grams (well, nearly) - so 1 mole of N2 weighs 28g, as you say. 1 molecule of N2 weighs around 10-26kg. 1 mole is usually written 1mol, which is the SI abbreviation. Is it possible that there's a mol(ecule) and mol(e) mix-up?

    Edit: By the way, I like "quite different". You have a way with understatement.
  4. Jul 23, 2012 #3


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    Exactly, it is the (current) definition of 1 mole.

    There are ideas to fix the avogadro constant, but this would include a re-definition of the atomic mass unit and change the atomic weights (at the 8. decimal place or something like this). 1 mole would stay the same.
  5. Jul 23, 2012 #4


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    Yes - apologies, I was confused. The mass in grams of a mole of anything is numerically equal to the atomic/molecular weight in Daltons. However, the atomic weight is not the same as the number of protons plus the number of neutrons in general because of (a) the mass defect and (b) the isotopic mix.
  6. Jul 23, 2012 #5
    there is also the whole binding energy with mass/energy equivilence to think about.

    for those reasons we have fusion and fission energy production.
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