Lec-05-Chap-02-2 - CHE 131 Lecture 5, 2/1/2006 What is the...

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CHE 131 Lecture 5, 2/1/2006 1 Molar Mass and The Periodic Table Lecture five (Chapter two) Benjamin S. Hsiao Department of Chemistry Stony Brook University What is the actual mass of a 12 C atom? Experimentally it is found to be: 1.9927 x 10 -26 kg This is a very inconvenient number so a new mass unit, the atomic mass unit (amu) has been defined. The amu is defined as 1/12 of the mass of a 12 C atom. An amu is also called a Dalton. 1 amu = 1/12 x 1.9927 x 10 -26 kg = 1.6605 x 10 -27 kg The mass of a 12 C atom is thus 12 amu exactly . C 6 12 98.9% C 6 13 1.1% The mass of a 12 C atom is 12 amu exactly by definition. The mass of a 13 C atom is 13.00335 amu from a mass spectroscopy experiment. What is the mass of an average C atom? .989 x 12 amu + .011 x 13.003 amu 12.011 amu But this is a very small mass. Chemists seldom deal with single atoms. Instead they deal with atoms in gram quantities. What is the conversion factor from amu to grams? It is a special number called Avogadro’s Number. N = 6.022142 x 10 23 The mass of a 12 C atom is 12 amu exactly by definition.
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CHE 131 Lecture 5, 2/1/2006 2 The Mole • a unit of measurement, quantity of matter present • Avogadro’s Number 6.022 × 10 23 particles • Latin for “pile” Molar Mass The mass of one mole of any object is the mass of 6.02 x 10 23 objects.
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Lec-05-Chap-02-2 - CHE 131 Lecture 5, 2/1/2006 What is the...

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