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84
Chapter Three
Stoichiometry
EXAMPLE
3.5
:==
Fragments
of cobalt
metal.
In this case, the term 12.01 limits the
number of significant
figures.
A substance's
molar mass is the mass in
grams of 1 mole of the substance.
Reality
Check:
Note that 5.68 mg of silicon is clearly much less than 1 mol of silicon
(which has a mass of 28.09 g), so the final answer
of 1.22
X
10
20
atoms (compared
with
6.022
X
10
23
atoms) is in the right direction.
SEE
EXERCISE
3.43
Cobalt (Co) is a metal that is added to steel to improve its resistance
to corrosion.
Calcu
late both the number
of moles
in a sample
of cobalt
containing
5.00
X 10
20
atoms
and
the mass of the sample.
Solution
Note that the sample
of 5.00
X 10
20
atoms
of cobalt
is less than
1 mole (6.022
X 1023
atoms) of cobalt. What fraction
of a mole it represents
can be determined
as follows:
1 mol Co
5.00 X 10
20
~
X
23
=
8.30 X 10
4
mol Co
6.022
X
10
~
Since the mass of 1 mole of cobalt atoms is 58.93 g, the mass of 5.00
X
10
20
atoms can
be determined
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 Spring '10
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 Organic chemistry, Atom, Stoichiometry

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