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Amu - 1 mole the number of atoms in 12 g of 12 C 1 u ...

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C1403 Kathleen Kristian Fall 2008 A Note on Mass Units At the end of recitation, there was a question about the relationship between grams or kilograms and amu or u (as Professor Valentini said, in this class we will use amu and u interchangeably because the difference is too small to matter at our level of precision). Specifically, one of you wondered where the conversion factor came from. The relationship given in your book is that 1 u = 1.661 x 10 -27 kg = 1.661 x 10 -24 g. You can quickly verify that the number in grams, 1.661 x10 -24 , is equal to 1/N A (the reciprocal of Avogadro’s number). The following calculation is meant to show you where this conversion factor came from. We have two definitions:
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Unformatted text preview: 1 mole ≡ the number of atoms in 12 g of 12 C. 1 u ≡ one-twelfth of the mass of one 12 C atom. From the second definition, we conclude that the mass of one 12 C atom is 12 u. Assuming that we have counted the number of 12 C atoms in 12 g of 12 C (it can be done by various techniques), and that this has in fact established that 1 mole = 6.022 x 10 23 particles, we have: # g u = 12 g 12 C 1 mole 12 C " # $ % & ’ 1 mole 12 C 6.022 x 10 23 12 C " # $ % & ’ 1 12 C 12 u " # $ % & ’ # g u = 1 6.022 x 10 23 " # $ % & ’ = 1.661 x 10 ( 24 g u = 1.661 x 10 ( 27 kg u All of this information is in the textbook and the slides but not explicitly written out....
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