lecture4 - Wed. Sept. 10, 2003 Todays Lecture Stoichiometry...

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1 1 Wed. Sept. 10, 2003 Today’s Lecture Stoichiometry • 3.1 Atomic Masses • 3.2 The Mole • 3.3 Molar mass • 3.4 Composition of Compounds • 3.5 Determining Formulae • 3.6 Chemical Equations 2 Atomic Mass by Mass Spectrometry – separates ions by mass-to-charge ratio 03_34 Slits Sample Positive ions Electron beam Heating device to vaporize sample Ion-accelerating electric field Accelerated ion beam Magnetic field Detector plate Least massive ions Most massive ions Fig. 3.1 3 Mass Spectrum of Mercury 4 Mass Spectrum of chlorine (Cl 2 ) 69 70 71 72 73 74 75 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Relative Intensity Mass Number 35 Cl: 75.53% and 37 Cl: 24.47%
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2 5 • Atomic masses: – relative scale of atomic masses is based on 12 C – the isotope, 12 C, is assigned a mass of exactly 12 atomic mass units (12 amu) – all other atomic masses are based on this standard – measure mass ratios: for 13 C: 13 C/ 12 C = 1.0836129 • mass of 13 C = 1.0836129 x 12 amu = 13.003355 amu 1 amu = 1.66056 x 10 -27 Kg 6 – Why is the atomic mass (weight) of C listed as 12.01? – C has 3 isotopes: 12 C, 13 C, and 14 C 14 C is negligibly small – average atomic mass of C: Isotope Mass (amu) Natural Abundance 12 C 12 (exactly) 98.89 % 13 C 13.00336 1.11 % amu 01 . 12 143 . 0 87 . 11 ) amu 003355 . 13 ( 100 11 . 1 ) amu 12 ( 100 89 . 98 = + = = + 7 • The Mole: – Chemistry involves vast numbers of atoms – unit used for counting atoms – 1 mol = # of C atoms in exactly 12 g of pure 12 C • 1 mol = 6.022 x 10 23 atoms Avogadro’s Number • recall: 12 C atom = 12 amu 6.022 x 10 23 amu = 1g The mass of 1 mole of any element is equal to its atomic mass expressed in grams – Example: • C = 12.01 amu • 1 mol C = 12.011 g 8 • Molar mass (or molecular weight): – The molar mass of a substance is the mass in g of 1
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This note was uploaded on 01/16/2010 for the course CHEM CHEM 121 taught by Professor Peggy during the Spring '09 term at Simon Fraser.

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lecture4 - Wed. Sept. 10, 2003 Todays Lecture Stoichiometry...

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