Finding the Mass of a
compound
Formula Mass
• Add up all the atomic masses in the
compound’s formula
Also called molecular mass if the compound
is covalently bonded (forms molecules)
(Remember, ionic compounds have no
molecules)
Formula mass
of NaCl:
Look up atomic mass of Na:
23 amu
Look up atomic mass of Cl:
35.3 amu
Formula mass
of NaCl:
Add the atomic masses together:
23 amu + 35.5 amu = 58.5 amu
Formula mass
of K2CO3:
This time you have:
two K
one C
three O
Formula mass
of K2CO3:
Multiply the atomic mass by the number of
atoms of that element in the formula
K
C
O
39.1
12.0
16.0
x 2=
x 1=
x 3=
78.2 amu
12.0 amu
48.0 amu
Formula mass
of K2CO3:
Now add the total mass for each element in
the compound
K
39.1 x 2 = 78.2 amu
C
12.0 x 1 = 12.0 amu
O
16.0 x 3 = 48.0 amu
+ ________
Formula Mass :
138.2 amu
Now you try it for:
Mg (NO3)2
Now you try it for:
Mg (NO3)2
Element
Mg
N
O
# of atoms
1
2
6
Now you try it for:
Mg (NO3)2
Element
Mg
N
O
# of atoms
1
2
6
Atomic mass
24.3 amu
14.0 amu
16.0 amu
Now you try it for:
Mg (NO3)2
Mg
N
O
1 x 24.3 = 24.3 amu
2 x 14.0 = 28.0 amu
6 x 16.0 = 96.0 amu
Mg (NO3)2
Mg
N
O
1 x 24.3 = 24.3 amu
2 x 14.0 = 28.0 amu
6 x 16.0 = 96.0 amu
+_______
148.3 amu
Gram Formula Mass (GFM)
is the formula mass expressed in grams
instead of in amu
This is a much more useful and practical unit
Follow the same steps as formula mass,
except your answer will have units of
grams, not amu
Formulas of Hydrates
A HYDRATE is a an ionic compound that
has water molecules attached to its crystal
structure in the solid phase.
These water molecules are weakly attached
to the crystal, and are easily removed if
the crystals are heated. Once the water is
removed, the compound is called
ANHYDROUS (without water).
Formulas of Hydrates
Na2CO3 • 10 H2O
This is a sodium carbonate crystal with ten
water molecules attached to every one
sodium carbonate
The • is the symbol that “attaches” the
ten water molecules to the sodium
carbonate. Think of it as a “+”
Finding GFM of Hydrates
Na2CO3 • 10 H2O
Type
Na
C
O
H2O
# of atoms
2
1
3
10
atomic mass
23
12
16
2 + 16 = 18
Finding GFM of Hydrates
Na2CO3 • 10 H2O
Na
C
O
H2O
2 x 23 =
1 x 12 =
3 x 16 =
10 x 18 =
46
12
48
+ 180
286 g
You try it!
Find the GFM of the hydrated crystal
CaSO4 • 2H2O
You try it!
CaSO4 • 2H2O
Ca
S
O
H2 O
40
32
16
18
x
x
x
x
1
1
4
2
=
=
=
=
40
32
64
+ 36
172 g