Atomic Weight, Molecular
Weight, Formula Weight and
Molar Mass
loosely based on Chap 4 Sec 1
of Jespersen 6th edition
Dr. C. Yau
Spring 2014
1
Atomic Weight or Atomic Mass
Examine the periodic table. What is the
atomic weight (AW) of carbon?
The atomic weight of C is
6
C
12 .01
or 12.01 u/atom
It means that each C atom weighs
12.01 u.
(In this class you should make it a habit to
look up AW to 4 sig. fig.)
2
Molecular Weight
or Molecular Mass
The AW of C is 12.01 u/atom
The AW of O is 16.00 u/atom.
Therefore, the mass of one molecule of CO2
would be 12.01 + 2(16.00) = 44.01 u/molecule.
(Always round to 4 sig. fig.)
We call this the molecular weight (MW) or
molecular mass.
What is the MW of methane (CH4)?
Ans. 16.04 u/molecule
3
Formula Mass
If we add up the atomic mass of Na and
Cl we get 22.99 + 35.45 = 58.44.
What is wrong with saying…
the molecular weight of NaCl is
58.44 u/molecule?
4
Formula Mass
For molecular substances, the smallest
unit is the molecule.
For ionic substances, the smallest unit
is the "formula unit" abbreviated f.u.
e.g. If you pluck out the smallest unit
out of water, you would have a water
molecule.
5
Formula Mass
When you pluck out the smallest unit of
table salt you would have a formula unit
of NaCl.
Na+ClNa+ClNa+Cl
ClNa+ClNa+ClNa+
Na+ClNa+ClNa+Cl
Na+Cl-
We often simply write
NaCl, but it is
understood that
Na is Na+ and Cl is Cl-.
There are no molecules in an ionic compound!
6
AW, MW and FW
Summary:
Atomic Weight (AW) = mass of one atom
(for atoms)
in units of amu/atom
Molecular Weight (MW) = mass of one molecule
(for molecular substances)
in units of amu/molecule
Formula Weight (FW) = mass of one f.u.
(for ionic compounds)
in units of amu/f.u.
7
Calculations of AW, MW and FW
Calculate the formula weight of…
(and give the proper units)
cobalt(III) sulfide
ammonium sulfate
Calculate the molecular weight of…
(CH3CH2)3N
8
Calculate the formula weight of…
Na2SO42H2O
This is known as a hydrate.
The dot in the formula does NOT mean
you multiply anything.
The dot means there are 2 water
molecules attached to each f.u. of
Na2SO4.
What is its formula weight?
9
The Mole Concept
One mole is defined as the number of
atoms in exactly 12 g of C-12 atoms.
Remember that C-12 refers to the
isotope….
exactly 12 g C
12
C
6
6.022x1023 C atoms
It turns out that there are 6.022x1023 C
atoms in those 12 grams. So….
1 mole = 6.022x1023 MEMORIZE THIS NOW!
(not an exact number, but in 4 sig. fig.)
10
The Mole Concept
1 mole = 6.022x1023
This number is known as
Avogadro's number
Note: Mole is abbreviated mol
NOT m (which stands for meters)
Note: It is a NUMBER.
It is NOT a mass.
It does NOT have the unit of grams.
11
The Mole Concept
We all know…
1 dozen eggs = 12 eggs
1 dozen atoms = 12 atoms
In the same way, we use the word
"mole"…
1 mole eggs = 6.022x1023 eggs
Obviously that is a VERY big number of
eggs. There would be no occasion to
be talking about a mole of eggs.
When do you we use "moles?"
12
The Mole Concept
1 mol atoms = 6.022x1023 atoms
1 mol molecules = 6.022x1023 molecules
1 mol electrons = 6.022x1023 electrons
The "mole" is not a mass itself, just a
number of items.
Why do we bother with such a strange
number?
Let us take a look at the periodic table.
13
The Mole Concept
The atom is a very, very small particle.
You can fit 7000 atoms on the dot of an i.
It is too small to see with the naked eye,
and too small to hold in your hand or
weigh on the balance.
We must use a much larger quantity of
atoms: a mole of atoms.
14
The Mole Concept
47
Ag
107 .87
From the periodic table, we can get
the atomic weight for Ag. This
mass can be given in two units:
107.9 u per atom
or
107.9 g per mole of atoms
One atom of silver weighs 107.9 u, which is
0.0000000000000000000001079 gram
Get the picture? You cannot weigh a single atom
of silver, but you CAN weigh a mole of Ag atoms.
15
107.9 g is about 3.8 ounces.
The Mole Concept
From the periodic table we see that…
Ag is 107.9 g/mol
Cu is 63.55 g/mol
Cu
Ag
What does 107.87 g of Ag have in common with
63.55 g of Cu?
They both contain the same number of atoms:
6.022x1023 Ag atoms and
6.022x1023 Cu atoms,
in other words, 1 mole Ag and 1 mol Cu.
16
The Mole Concept
Earlier we were talking about the
AW, MW and FW. Now we are going to add
one more….
MM = molar mass
Whenever you see "molar" you should think
"per mole."
Molar mass = mass per mole (for 1 mol)
with units of g/mol
KNOW THIS WELL! SEE THIS IN YOUR
DREAMS….AND NIGHTMARES!
17
Molar Mass (MM)
The atomic mass from the periodic
table can be used with two units:
amu/atom or g/mole (or g mol-1)
REMEMBER: Whenever you see a
fractional unit
(such as amu/atom, g/mol, g/mL)
you should think "conversion factor"!
The g/mol gives you the link between
MASS AND # of MOLES.
18
Molar Mass (MM)
The FW of
Co2(SO4)3 is 406.7 amu/f.u.
Its MM = 406.7 g/mol
2(58.93)+3(32.07)+12(16.00) = 406.7
We had calculated the MW of…
(CH3CH2)3N to be 101.2 amu/molecule
Its MM = 101.2 g/mol
2x3C + 5x3H + N
= 6(12.01) + 15(1.008) + 14.01 = 101.2
19
Molar Mass (MM)
Note that we no longer need to
distinguish between f.u. and
molecules in the units of MM.
It is understood that for Co2(SO4)3
its MM = 406.7 g/mol of f.u.
and for (CH3CH2)3N
its MM = 101.2 g/mol of molecules.
20
Use of MM in Dimensional
Analysis
Conversion factors from MM:
e.g. MM (N) = 14.01 g/mol
 14.01 g N   1 mol N 


 or 
 1 mol N   14.01 g N 

 

e.g. MM (HCl) = 36.45 g/mol
 36.46 g HCl   1 mol HCl 


 or 
 1 mol HCl   36.46 g HCl 

 

21
Use of MM in Dimensional Analysis
When do we use MM?
Whenever you see a fractional unit (such
as g/mol or g mol-1) you should think
"conversion factor"!
MM is the link between mass & moles
mass
moles of particles
moles of atoms
moles of f.u.
moles of molecules
e.g.
mol Cu
mol CuCl2
mol PBr3
22
Use of MM
Example 4.1 p.109
Titanium(IV) oxide is one of the best
sunscreens because it completely blocks
ultraviolet radiation from reaching the
skin. In an experiment to prepare TiO2,
we start with 23.5 g sample of titanium.
How many moles of Ti do we have?
23
Example 4.2 p.110
We need 0.254 mol of iron(III) chloride for
a certain experiment. How many grams
do we need to weigh?
Do Practice Exercises 4.1 & 4.2 on p.110
(Note: Ans to Pract. Exercises are at the back of the
book.)
24
Use of Avogadro's Number in
Dimensional Analysis
Conversion factors
from Avogadro's number:
 1 mol X
  6.022x10 23 X 

 or
 6.022x10 23 X   1 mol X


 

Note that X can be anyTHING, such as CO2,
K2SO4, electrons, Na+, etc….
as long as it is EXACTLY the same on top as on
the bottom.
BUT it cannot be a unit, such as g or mL.
25
Use of Avogadro's Number in Dimensional Analysis
You use Avogadro's number ONLY if you
have to go between
the macroscopic and
the particulate (sub-microscopic) levels.
particulate level
(conceptual,
non-observable)
atoms
f.u.
molecules
u
macrosopic level
(observable)
moles of atoms
moles of f.u.
moles of molecules
g, mg, lbs, oz
26
Use of Avogadro's Number
Example 4.3 p.111
Tungsten wire is the filament inside most
incandescent light bulbs. In a typical light
bulb, the tungsten filament weighs 0.635 g.
How many atoms of tungsten are there in such
a light bulb flame?
First ask yourself... “Do I need Avog. #?”
27
Example 4.4 p.112
Carbon tetrachloride was used as a drycleaning fluid until it was found to be
carcinogen. What is the average mass
in grams of one molecule of carbon
tetrachloride (CCl4)?
“Do I need Avog. #?”
28
Use of Avogadro's Number
Example
What is the mass in grams of 2.01 mole of NaCl?
Example
How many anions are in 7.82 g of MgCl2?
Example
What is the weight in grams of 158 molecules of
CO2?
REMEMBER! ASK YOURSELF…
DO I NEED AVOGADRO'S NUMBER IN THIS PROBLEM?
Do Practice Exercises 4.3 & 4.4 on p.112
29