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Chapter 3
Stoichiometry
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2. Measurement of Atomic Mass
Most elements occur in nature as mixture of isotopes; thus atomic masses are usually average values.
The modern system of atomic masses, instituted in 1961, is based on 12C as the standard. In this system 12C is assigned a mass of exactly 12 atomic mass units (amu), and the masses of all other atoms are given relative to this standard.
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Mass spectrometer
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Atomic Mass of Carbon
12C and 13C are analyzed in a mass spectrometer, the ratio of their masses is found to be: Mass 13C/Mass 12C=
Mass of 13C=( )(12amu)= amu
The natural carbon is composed of 98.89% 12C atoms and 1.11% 13C atoms.
(0.9889)(12 amu)+(0.0111)( amu)=12.01 amu
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Peaks of neon injected
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6. Bar graph of neon injected
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7. Percent Composition of Compounds
1 mole C2H5OH
Mass of C=2 mol X g/mol= g
Mass of H=6 mol X g/mol=6.048 g
Mass of O=1 mol X g/mol= g
Mass of 1 mol of C2H5OH= g
Mass percent of C=(24.022/46.069)x100%=52.144%
Mass percent of H=(6.048/46.069)X100%=13.13%
Mass percent of O=(15.999/46.069)X100%=34.728%
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8. Determining the Formula of a Compound
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Chemical Equations
Need to know the reactants and products
Balancing a chemical equation
Specify the physical states of the reactants and products
Stoichiometric calculations
Calculations involving a limiting reaction
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