Chapter 7

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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
Average Atomic Mass and the Periodic Table
You have learned that you can use atomic masses
on the periodic table to find the molar mass of
elements. Many of these values on the
periodic table are close to whole numbers.
However, most atomic masses are written to at
least three places past the decimal.
Most Elements Are Mixtures of Isotopes
You remember that isotopes are atoms that have
different numbers of neutrons than other atoms of
the same element do. So, isotopes have
different atomic masses. The periodic table
reports a average atomic mass weighted average of
the atomic mass of an elements isotopes.
A weighted average takes into account the
relative importance of each number in the
average. Thus, if there is more of one isotope in
a typical sample, it affects the
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
Remember from Chapter 3 the formula to calculate
the average atomic mass
(mass
of atom1 x quantity1) (mass of atom2 x
quantity2) .. Average Atomic Mass

100
Calculate the average atomic mass of Copper using
the information below. atomic mass of a Cu-63
atom 62.94 amu abundance of Cu-63 69.17
atomic mass of Cu-65 64.93 amu abundance of
Cu-65 30.83
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
(62.94 x 69.17) (64.93 x 30.83) Mass of
Copper 100
63.55 amu
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
Chemical Formulas and Moles
Until now, when you needed to perform molar
conversions, you were given the molar mass of
compounds in a sample. Where does this molar mass
of compounds come from? You can determine the
molar mass of compounds the same way that you
find the molar mass of individual elements by
using the periodic table.
Formulas Express Composition
You have learned that covalent compounds, such as
water and hexachloroethane, consist of molecules
as units. Formulas for covalent compounds show
both the elements and the number of atoms of
each element in a molecule. Hexachloroethane has
the formula C2Cl6. Each molecule has 8 atoms
covalently bonded to each other. Ionic compounds
arent found as molecules, so their formulas do
not show numbers of atoms. Instead, the formula
shows the simplest ratio of cations and anions.
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
Formulas Are Used to Calculate Molar Masses
A formula tells you what atoms (or ions) are
present in an element or compound. So, from a
formula you can find the mass of a mole of the
substance, or its molar mass. To do this you use
the formula to tell you how many of which Atoms
an to look up there mass from the periodic table.
Find the molar mass for each of the following
compounds a. CsI b. C12H22O11 c.
HC2H3O2 d. CaHPO4 e. I2 f. Mg3(PO4)2
259.8 g/mol
342.0 g/mol
60.0 g/mol
136.1 g/mol
253.8 g/mol
262.9 g/mol
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas
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Chapter 7 The Mole and Chemical Composition Sec
2 - Relative Atomic Mass and Chemical Formulas