Download The Mole, Molar Mass and Avogadro`s Number

12/11/2013
The Mole, Molar Mass and
Avogadro’s Number
Mass Relationships in Chemical Reactions
Look at the “atomic masses” on
the periodic table. What do
these represent?
Atomic mass is the mass of
the atom in atomic mass units
(amu). One amu is defined as
a mass equal to one-twelfth the
mass of one carbon-12 atom
Carbon-12 is the carbon isotope
that has 6 protons and 6
neutrons
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Carbon’s atomic mass on the
periodic table is 12.01 amu
not 12.00 amu. Why the
difference?
12.01 amu represents the
average mass of the naturally
occurring mixture of isotopes
of carbon
Two problems
1. Atomic masses do not convert easily to grams
2. They can’t be weighed (they are too small)
The Mole
A mole is defined as the #
of atoms of carbon-12 in
exactly 12 grams of
carbon-12
The mole is a counting unit.
It is used to compare the #
of atoms, molecules, ions,
in any substance
There are 6.02 x 1023 particles in one mole of
any substance. This number is called
Avogadro’s number (NA)
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1 dozen = 12 items
1 gross = 144 items
1 mole = 6.02 x 1023 items
One mole of carbon-12 has a mass of exactly 12
grams or 12 g/mol. This is defined as the molar
mass, the mass of one mole of units.
e.g. 1 mol Li = 6.94 g Li is
expressed as 6.94g/mol
The molar mass, molecular weight, or formula
weight equations used to determine the mass in
grams of one mole of a substance
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The molar mass of calcium phosphate, Ca3(PO4)2 =
(3 x 40.08 g/mol Ca) + (4 x 2 x 16.0 g/mol O) + (2 x 31.0
g/mol P) = 310.2 g/mol Ca3(PO4)2
Calculate molar masses (to 2 decimal places)
CaCl2
110.984 g/mol
(NH4)2CO3
98.088 g/mol
Pb3(PO4)2
811.546 g/mol
C6H12O6
180.162 g/mol
Avogadro’s, NA, relationship is used to convert from
the number of moles of a substance to the number
of atoms, ions, or molecules of a substance and vice
versa
(unit = mol-1)
These values may be used to determine the
number of atoms in a given mass of an element
MOLAR MASS
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or the mass of one atom, ion, or molecule
The subscripts within a chemical formula also
indicate the number of moles of that element or ion
in one mole of that compound.
(ratio = mol/mol)
These ratios are useful when determining the
number of moles or particles in a particular
compound.
MOLE to MOLE RATIO
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How many atoms of oxygen are in 3.5 grams of
calcium phosphate?
Once the number of moles of the substance is
known, you can determine the number of
molecules or formula units of the substance.
Molecules are found in covalent compounds. Ionic
compounds do not have molecules their smallest
subunits are called formula units.
How many formula units of calcium phosphate are
there in 0.211 moles of Ca3(PO4)2 ?
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PRACTICE
Calculate the number of moles in each of the following:
64.1 g of aluminum
850.5 g of ammonium phosphate
Calculate the mass of each of the following;
0.275 mol mercury
9.37x10-3 mol calcium chlorate
Calculate the amount in moles in each of the following;
8.08 x 1022 atoms of krypton
2.997 x 1025 atoms of oxygen in vanadium(V) oxide
Calculate the mass of the following:
3.01 x 1021 atoms of cobalt
1,20 x 1025 atoms of helium
Calculate the number of atoms in each of the following:
0.697 g of gallium
69.45 g of lanthanum
Determine the number of molecules in 0.0500 mol of
hexane, C6H14
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