9. Formula Mass Calculations
Formula mass is the total of all the atomic masses in a chemical formula.
To make these calculations easier, round all atomic mass numbers to the nearest whole number.
Sample Problem: calculate the formula mass of C2H5OH
Use atomic mass from the Periodic Table:
Set up your calculation this way:
element = (# of atoms) (atomic mass) = u
The units of formula mas are "atomic mass units" - u
 Formula Mass Calculation #1
Formula Mass Calculation #2
Formulas of Hydrated Crystals:
Hydrated crystals have water molecules chemically bonded to ions in the crystal. This occurs when a highly ionic substance crystallizes from a water solution. Although hydrated crystals are solids, the water molecules can be forced out of the crystal lattice by heating.
CuSO4 • 5H2O
The formula for "hydrated" copper (II) sulfate above indicates there are 5 water molecules for every copper (II) sulfate.
The formula mass calculation for
copper (II) sulfate pentahydrate looks like this:
|
|
Cu
|
-
|
1
|
X
|
64
|
=
|
64
|
|
S
|
-
|
1
|
X
|
32
|
=
|
32
|
|
O
|
-
|
4
|
X
|
16
|
=
|
64
|
|
H2O
|
-
|
5
|
X
|
18
|
=
|
90
|
|
Total 250 amu
|
 Formula Mass
 Hydrated Crystals
|
