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Lecture_4

# Lecture_4 - Today's Lecture Stoichiometry 3.1 Atomic Masses...

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1 Stoichiometry Today’s Lecture 3.1 Atomic Masses 3.2 The Mole 3.3 Molar Mass 3.4 Percent Composition of Compounds 1 Mass Spectrometry ± we cannot determine the mass of an individual atom just by adding up the masses of its subatomic particles - why? ± determining th em a s sof individual atoms is done by experiment ( 12 Ci sa s s ig nedam a s so fex a c t l y12a t om i c mass units (amu) and the masses of all other atoms are 03_34 Ion-accelerating electric field diagram of a mass 2 Slits Sample Positive ions Electron beam Heating device to vaporize sample

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2 Isotopic Masses by Mass Spectrometry ± e.g. when a sample of pure carbon is analyzed by mass spectrometry, the ratio of the mass of 13 Cto 12 Cisfound to be: ± mass of 13 C = (1.0836129) (12 amu) = 13.00335 amu exact number by mass of 12 C = 1.0836129 mass of 13 C 3 ± the isotopic masses of other atoms can be determined in a similar manner (isotopic mass is the mass, in amu, of an individual atom, relative to an arbitrarily assigned value of exactly 12 amu as the mass of the 12 Cisotope) definition Atomic Masses ± it is often more convenient to express the atomic mass of an element rather than the isotopic mass of each isotope of that element ± atomic mass is the average mass of the isotopes in the natural form of a given element (must take into account the natural abundance of each isotope) ± e.g. what is the atomic mass of naturally occurring carbon? ± the mass spectrum of pure carbon shows that 98.892% of carbon atoms are 12 C (the isotopic mass of 12 C i 4 C is assigned a value of 12 amu) and 1.108% are 13 C (the isotopic mass of 13 C is 13.00335 amu) (0.98892)(12 amu) + (0.01108)(13.00335 amu) = 11.867 amu + 0.1441 amu = 12.011 amu (the atomic mass of carbon is 12.011 amu)
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Lecture_4 - Today's Lecture Stoichiometry 3.1 Atomic Masses...

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