Transcript 6.3 The Gram Formula Mass gram molecular mass

6.3 The Gram Formula Mass
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You are always working with large numbers of
atoms even when you are working with very
small amounts of a substance.
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Because of this, it is easier to work with grams of
atoms
gram atomic mass (gam) – the atomic mass of
an element expressed in grams
Example: One gram atomic mass of carbon is
12.0 g
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6.3 The Gram Formula Mass
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You determine the gram atomic mass (gam) of a
substance by finding the atomic mass of a
substance.
What is the gam for: H, Fe, Hg, and S
1 gam H = 1.0 g H
 1 gam Fe = 55.8 g Fe
 1 gam Hg = 200.6 g Hg
 1 gam S = 32.1 g S
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6.3 The Gram Formula Mass
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The gram atomic masses of any two
elements must contain the same number of
atoms
A mole is defined as the amount of a substance
that contains as many representative particles as
the number of atoms in 12.0 g of carbon-12
(6.02 x 1023 representative units)
This means that the gram atomic mass is the
mass of one mole of atoms of any element.
 Example: 1 mol C = 1 gam C = 12.0 g C
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6.3 The Gram Formula Mass
12. What is the mass of 1 mole of each of the
following monatomic elements?
a. sodium
A: 23.0 g
b. selenium
A: 79.0 g
c. lead
A: 207.2 g
6.3 The Gram Formula Mass
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To determine the mass of a mole of a
compound you must know the formula of a
compound
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The formula tells you the number of atoms of
each element in a representative particle of that
compound
6.3 The Gram Formula Mass
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Example: sulfur trioxide, SO3, contains 1 atom
of S and 3 atoms of O
You can use the atomic masses of S and O to
determine the molar mass of SO3
 1 atom of S = 32.1 amu
 3 atoms of O = 3 x 16.0 = 48.0 amu
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molecular mass SO3 = 32.1 amu + 48.0 amu = 80.1 amu
You can substitute the unit grams for amu’s to
obtain the gram molecular mass of SO3
6.3 The Gram Formula Mass
gram molecular mass (gmm) – (for any
molecular compound) is the mass of one mole
of that compound
Example:
1 mol SO3 = 6.02 x 1023 molecules SO3 = 80.1 g SO3
* Gram molecular masses may be calculated directly
from gram atomic masses
6.3 The Gram Formula Mass
Example: What is the gram molecular mass of
dihydrogen dioxide, H2O2?
U: gmm of dihydrogen dioxide
K: Molecular Formula - H2O2 = 2 H atoms, 2 O atoms
1 mol H = 1.0 g H
1 mol O = 16.0 g O
S: 2 mol H x (1.0 g H/1 mol H) = 2.0 g H
2 mol O x (16.0 g O/1 mol O) = 32.0 g O
Gram Molecular Mass H2O2 = 2.0 g + 32.0 g = 34.0 g
6.3 The Gram Formula Mass
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Since the representative unit ionic compounds is
the formula unit and not a molecule, you cannot
represent these compounds with the gram
molecular mass unit.
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However, you can use the gram formula mass
(gfm), which is calculated in the same way.
6.3 The Gram Formula Mass
Example: Calculate the gram formula unit for
CaI2.
U: gfm of calcium iodide
K: Formula Unit – CaI2 = 1 Ca atom, 2 I atoms
S:
gfm = 40.1 g Ca + (2 x 126.9 g I) = 293.9 g CaI2
6.4 The Molar Mass of a Substance
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The terms gram atomic mass (gam), gram
molecular mass (gmm), and gram formula mass
(gfm) represent the mass of a mole of a
particular substance.
It is appropriate to use the term gram formula
mass, or just molar mass, to refer to the mass
of a mole of any element or compound.