Hydrated crystals have water molecules bonded to ions in the crystal. This occurs when a highly ionic substance crystallizes from a water solution.
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
|
Notice that water is treated as ONE substance in this calculation!
|
Although hydrated crystals are solids, the water molecules CAN be forced out of the crystal lattice by heating - forming an "anhydrous" crystal - meaning "without water".
anhydrous crystal
To find the percentage of this compound that is water, divide the total amu provided by water by the total amu of the compound.
90 / 250 = 0.36 X 100 = 36%
Copper (II) sulfate pentahydrate is 36% water by mass.
HINT: A major source of experimental error −
When heating the hydrated crystal, how can you be sure that all of the water has been removed?
The Material Safety Data Sheet for Copper (II) Sulfate.
