An Introduction…..
Background: Atomic masses
• Atomic Mass: Where can you find it?
• An atomic mass unit is defined as 1/12 the
weight of the carbon-12 isotope. The old symbol
was amu, while the most correct symbol is u (a
lower case letter u).
• The atomic mass of any element is the
“average” atomic mass and found on the
periodic table.
Background: Atomic masses
Two problems with atomic mass
1. Atomic masses do not convert easily to
grams
2. They can’t be weighed (they are too
small)
Therefore, atomic mass is not that
practical! And we need something that
is……..
The Mole
No, not a little critter but a number………
The Mole
• A counting unit!
• Similar to a dozen, except instead
of 12, it’s
602,000,000,000,000,000,000,000
• 6.02 X 1023 (in scientific notation)
often called Avogadro’s #
• This number is named in honor of
Amedeo Avogadro (1776 – 1856)
Here is how the International Union
of Pure and Applied Chemistry
(IUPAC) defines "mole:"
• The mole is the amount of substance of
a system which contains as many
elementary entities as there are atoms
in 0.012 kilogram of carbon-12. When
the mole is used, the elementary
entities must be specified and may be
atoms, molecules, ions, electrons, other
particles, or specified groups of such
particles.
What is Avogadro's Number
again?
one mole of ANYTHING contains
6.02 x 1023 entities.
In other words, there are 6.02 x 1023
anythings in one mole of anything!
Molar Mass
• The mass of one mole is called “molar mass”
• The unit is g/mol
• Equal to the numerical value from periodic
table, or add the mass of the atoms together
for a molecule
1 mole of C atoms
=
12.0 g/mol
1 mole of Mg atoms
=
24.3 g/mol
1 mole of O2 molecules
=
32.0 g/mol
Molar Mass of Compounds
• The molar mass (MM) of a compound is
determined the same way, except you add
up all the atomic masses for the molecule
Molar Mass of Compounds
Don’t forget the subscripts!!!!
Ex. Molar mass of CaCl2
1. Avg. Atomic mass of Calcium = 40.08g
2. Avg. Atomic mass of Chlorine = 35.45g
3. Molar Mass of calcium chloride =
40.08 g/mol Ca + (2 X 35.45) g/mol Cl
 110.98 g/mol CaCl2
20
Ca
40.08
17
Cl
35.45
Particles
Flowchart
(Atoms or
Molecules)
Divide by 6.02 X 1023
Multiply by 6.02 X 1023
Moles
Divide by
atomic/molar mass
from periodic table
Multiply by
atomic/molar mass
from periodic table
Mass
(grams)
Calculations
molar mass
Grams
Avogadro’s number
Moles
particles
Everything must go through
Moles!!!
Try it together:
• What is the molar mass (MM) of:
lead (II) nitrate
Pb(NO3)2
Pb 207.2 g/mol
N  14.0 g/mol x 2
O  15.99 g/mol x 6
207.2 + (14.0 x 2) + (15.99 x 6)
=
331.14 g/mol
Finding MM Practise:
• Find the molar mass of the following:
a)
b)
c)
d)
e)
f)
g)
PbSO4
Ca(OH)2
Na3PO4
(NH4)2CO3
C6H12O6
Fe3(PO4)2
Zn(C2H3O2)2
• Complete page 57 #16, 17, 18, 19
Day 2
Mass to Moles
How many moles of KBr are in a 34.91 g
sample?
Find MM: 39.09 +79.9 = 118.9 g/mol KBr
x mol = 34.91 g
x ___1 mol = 0.29 mol KBr
118.9 g
Moles to Mass
How many grams of CaCl2 are in a 0.450
mol sample?
Find MM: 40.07 +(35.4 x2) = 110.87g/mol CaCl2
x g = 0.450 mol
x 110.87 g = 49.89 g CaCl2
1 mol
Mass to particles
Magnesium and oxygen react to form MgO ,how
many molecules of MgO are contained in a 12.56g
sample of MgO ?
Find MM: 24.3 +15.99 = 40.29 g/mol MgO
x molecules = 12.56 g x 1 mol x 6.02 x 1023 molecules
40.29 g
= 1.88 x 1023
molecules of MgO
1 mol
Need to get atoms?
• Set up a conversion with atoms per
molecule
Ex: (NH4)2S
How many individual atoms are there in
this molecule?
2 + 8 + 1 = 11 atoms for 1 molecule
Mass to atoms
Magnesium and oxygen react to form MgO ,how
many atoms of MgO are contained in a 12.56g
sample of MgO ?
Find MM: 24.3 +15.99 = 40.29 g/mol MgO
x molecules = 12.56 g x 1 mol x 6.02 x 1023 molecules x 2 atoms
40.29 g
1 mol
= 3.75 x 1023 atoms
of MgO
1 molecule
Try it:
•
•
•
•
Pg 59 # 20
Pg 60 # 24
Pg 63 # 28 (formula units = molecules)
Pg 64 # 34