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4.S: The Mole and Measurements in Chemistry (Summary)
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The
mole
is defined as the number of atoms contained in
exactly
12 grams of carbon-12 (the isotope ). There are 6.0221415 x 10
23
particles in a mole. Remember, a
mole
is just a number (like
dozen
) and you can have a mole of
anything.
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The concept of a mole is based on
Avogadro’s Hypothesis
(
equal volumes of all gases at the same temperature and pressure contained the same number of molecules
) and the number of particles in a mole (6.0221415 x 10
23
) is commonly referred to as
Avogadro’s number
(typically rounded to 6.02 x 10
23
for most calculations).
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Because atomic masses, and the number of particles in a mole, are both based on the isotopic atomic mass of the isotope carbon-12,
the
mass of any substance expressed in atomic mass units is numerically equal to the
molar mass
of the substance in grams
per
mole
. Thus,
exactly
12 grams of carbon-12 contains
exactly
a mole of carbon atoms; likewise, 31.9988 grams of O
2
contains 6.02214 x 10
23
oxygen molecules (note,
six
significant figures), etc.
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To convert the number of
moles
of a substance into the
mass
of a substance, you simply need to multiply (
moles
x
molar mass
).
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To convert the
mass
of a substance into the number of
moles
, you simply need to
divide
the
mass
by the
molar mass
.
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To convert the number of
moles
of a substance into the number of
particles
of that substance, you simply need to multiply (
moles
x
Avogadro’s number
).
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The
percentage composition
of a compound, simply tells us what
percent
of the total mass arises from each element in the compound. To do the calculation, simply take the atomic mass of the element in question and divide it by the molar mass of the molecule.
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The
empirical formula
for a compound is the
lowest whole-number ratio
of the elements in that compound. For example, the molecular formula for glucose is C
6
H
12
O
6
, but the simplest whole-number ratio of the elements in glucose is CH
2
O.