Molar Mass
Vocabulary
•average atomic mass
•molar mass
Mona Lisa, Fernando Botero, 1977.
Colombian.
What does “mole” mean?
A mole is a number.
It is 6.02x1023.
A mole of anything is
6.02x1023 units of that thing.
Comparison
So if you have a mole of mice, how
many mice do you have?
6.02 x 1023 mice!!!
If you have a mole of elephants, how
many elephants do you have?
6.02x1023 elephants!!!
A mole is a mole!
A mole of mice and a mole of
elephants have the same number of
animals: 6.02 x 1023.
But do they have the same mass?
NO!!! A mole of elephants weighs
substantially more than a mole of
mice!
The same is true with atoms. A mole
of a light element like hydrogen will
have much less mass than a mole of a
heavy element like gold.
hydrogen atom
gold atom
Molar Mass to the rescue
The mass of one mole of every
element is on the periodic table. The
average atomic mass is the mass of
one atom of an element in amu, but
that number is also called the molar
mass and is the mass of one mole of
an element in grams.
Average atomic mass = mass of one atom in amu
Same number!!
Molar mass = mass of one mole in grams
Try these:
 What is the molar mass of carbon?
 12.01 g/mol meaning 12.01 g C = 1.000 mol C
 What is the molar mass of gold?
 196.97 g/mol meaning 196.97 g Au = 1.0000 mol Au
 What is the molar mass of tungsten?
 183.84 g/mol meaning 183.84 g W = 1.0000 mol W
What about compounds?
So how do you think we can
determine the molar mass of a
compound like carbon dioxide, CO2?
Simply add the molar masses of
each atom making up the
compound. In this case, that means
1 carbon atom and 2 oxygen atoms.
What about compounds?
To find the molar mass of CO2:
Element
Molar mass
x Number of atoms
Total
carbon
12.01 g/mol
x1
12.01 g/mol
oxygen
16.00 g/mol
x2
32.00 g/mol
Total
------------
-----------
44.01 g/mol
So the molar mass of CO2 is 44.01 g/mol,
meaning 1 mol of CO2 has a mass of 44.01 g.
You try:
What is the molar mass of
calcium chloride?
Calcium chloride is CaCl2
Element
Molar mass
x Number of atoms
Total
calcium
40.08 g/mol
x1
40.08 g/mol
chlorine
35.45 g/mol
x2
70.90 g/mol
Total
------------
-----------
110.98 g/mol
The use of molar mass
Molar mass is important because it
allows us to convert between the
mass of a substance and the moles
of a substance.
Let’s try using molar mass in a
dimensional analysis problem
What is the mass of 4.5 moles of gold?
(Remember: the molar mass of gold from the
periodic table is 196.97 g/mol.)
4.5 mol Au
x 196.97 g Au = 890 g Au
1 mol Au
Start with given Use appropriate
amount you are conversion
trying to convert. factor.
Write answer
with units and
rounded to
proper sig figs.
Let’s try another problem
How many moles of Li2O will have a mass of 392 g?
(Remember: find the molar mass of Li2O.
392 g Li2O
x 1 mol Li2O = 13.1 mol Li2O
29.88 g Li2O
Start with given Use appropriate
amount you are conversion
trying to convert. factor.
Write answer
with units and
rounded to
proper sig figs.