Unit 5.5 Percent of Water in a Hydrate
Why does the color change? If you look at a typical bottle of
copper sulfate, it will be a bluish-green. If someone tells you
that copper sulfate is white, you won’t believe them. You are
both right; it just depends on the copper sulfate. Your bluegreen copper sulfate has several water molecules attached
to it while your friend’s copper sulfate is anhydrous (no
water attached). Why the difference? The water molecules
interact with some of the d electrons in the copper ion and
produce the color. When the water is removed, the electron
configuration changes and the bluish-green color will
disappear.
Percent of Water in a Hydrate
Many ionic compounds naturally contain water as part of the crystal lattice structure. A hydrate is a
compound that has one or more water molecules bound to each formula unit. Ionic compounds that
contain a transition metal are often highly colored. Interestingly, it is common for the hydrated form of
a compound to be of a different color than the anhydrous form, which has no water in its structure. A
hydrate can usually be converted to the anhydrous compound by heating. For example, the anhydrous
compound cobalt (II) chloride is blue, while the hydrate is a distinctive magenta color.
On the left is
anhydrous cobalt (II)
chloride (CoCl2), on the
right is the hydrated
form of the compound
called cobalt (II)
chloride hexahydrate
.
CoCl2 6H2O.
The hydrated form of cobalt (II) chloride contains six water molecules in each formula unit. The name
of the compound is cobalt (II) chloride hexahydrate and its formula is CoCl2•6H2O. The formula for
water is set apart at the end of the formula with a dot, followed by a coefficient that represents the
number of water molecules per formula unit.
It is useful to know the percent of water contained within a hydrate. The sample problem below
demonstrates the procedure.
To find the % H2O use:
% H2O =
(
mass in grams of H2O attached to hydrate
mass in grams of entire hydrate compound
)
(100)
Sample Problem One: Percent of Water in a Hydrate
Find the percent water in cobalt (II) chloride hexahydrate, CoCl2•6H2O.
Step 1: List the known
unknown
Co = 58.93 g
% H2O = ?
(2) (35.45 g Cl) = 70.90 g
(12) (1.02 g H) = 12.12 g
The molar mass of the hydrate is the
(6) (16.00 g O) = 96.00 g
molar mass of the CoCl2 plus the mass
g
Total mass Hydrate = 237.95 /mol
of water.
(12) (1.02 g H) = 12.12 g
The mass of water in the hydrate is
(6) (16.00 g O) = 96.00 g
the coefficient (6) multiplied by the
molar mass of H2O.
Total mass H2O = 108.12 g
Calculate the % H2O by dividing the mass of H2O in 1 mole of the hydrate by the molar mass of the
hydrate and multiplying by 100%.
Step 2: Calculate.
% H2O =
(
)
237.95 g hydrate
108.12 g H2O
(100)
=
45.44 % H2O
Step 3: Think about your result.
Nearly half of the mass of the hydrate is composed of water molecules within the crystal.
Summary

A hydrate is a compound that has one or more water molecules bound to each formula unit.

An anhydrous compound is one which has no water in its structure.

The % H2O can be calculated using:
% H2 O =
( mass in grams of entire hydrate compound )
mass in grams of H2O attached to hydrate
(100)
Review
1.
What is a hydrate?
2.
How can you convert a hydrate to an anhydrous compound?
3.
What does hexahydrate mean?
4.
What is the % H2O in MgSO4 • 7 H2O
Answers
1.
A hydrate is a compound that has one or more water molecules bound to each formula unit.
2.
Heating a compound can drive off the H2O leaving an anhydrous (without H2O) compound.
3.
Hexahydrate means 6 H2O attached.
4.
% H2O in MgSO4 • 7 H2O
Mg = 24.31 g
S = 32.07 g
(4) (16.00 g O) = 62.00 g
(14) (1.01 g H) = 14.14 g
(7) (16.00 g O) = 112.00 g
126.14 g H2O
Total mass hydrate = 244.34 g/mol
126.12 g H2O
244.34 g hydrate
x 100 = 51.62 % H2O