1
Composition
Stoichiometry
Principles of General Chemistry, 2
nd
ed. By M. Silberberg
Chemistry, 8th ed.
by W. Whitten, R. Davis, R., M. L. Peck, and G. Stanley.
2
Lecture
•
The Mole
•
Atomic Weights
•
Formula Weights, Molecular Weights and Moles
•
Percent Composition
•
Derivation of Formulas from Elemental
Composition
•
Determination of Molecular Formulas
•
Some Other Interpretations of Chemical Formulas
•
Purity of Samples
The Atomic Weights
•
If we define the mass of
12
C as exactly 12 atomic mass
units (amu), then it is possible to establish a relative
weight scale for atoms.
–
1 amu = (1/12) mass of
12
C by definition
3
•
Atomic Weight  weighted average
of the masses of the constituent
isotopes of an element.
•
Example.
Naturally occurring Cu consists of 2
isotopes.
It is 69.1%
63
Cu with a mass of 62.9
amu, and 30.9%
65
Cu, which has a mass of 64.9
amu.
Calculate the atomic weight of Cu to one
decimal place.
4
The Atomic Weights
•
Example.
The atomic weight of boron is 10.811
amu.
The masses of the two naturally occurring
isotopes
5
10
B and
5
11
B, are 10.013 and 11.009
amu, respectively.
Calculate the fraction and
percentage of each isotope.
5
The Atomic Weights
6
The Mole
•
A number of atoms, ions, or molecules that is large
enough to see and handle.
•
A mole = number of things
–
Just like a dozen = 12 things
–
One mole = 6.022 x 10
23
things
•
Avogadro’s number = 6.022 x 10
23
–
Symbol for Avogadro’s number is N.
One mole (1 mol) contains 6.022 x 10
23
entities (to four
significant figures)
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7
Interconverting Moles, Mass, and Number of
Chemical Entities
Mass (g) = no. of moles
x
no. of grams
1 mol
No. of moles = mass (g)
x
no. of grams
1 mol
No. of entities = no. of moles
x
6.022 x 10
23
entities
1 mol
No. of moles = no. of entities
x
6.022 x 10
23
entities
1 mol
g
No. of moles = mass (g)
x
no. of grams
1 mol
mol
8
•
Example:
Calculate the mass of a single Mg
atom in grams to 3 significant figures.
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 Spring '07
 SIAO
 Atom, Mole, Stoichiometry, Molecule, Avogadro constant, Interconverting Moles

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