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102 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
10.5 The Formula for a Hydrate
Many compounds, particularly ionic compounds, incorporate a specific number of water molecules into their crystals when they
crystallize from aqueous solution. Compounds that include water in
their crystal structures are called hydrates. The formula for a
hydrate is written by adding the formula for water to the formula
for the compound. The number of water molecules included per
formula unit is indicated by a coefficient. The formula for the blue
crystals of copper(II) sulfate is CuSO 4 и 5H 2 O, indicating that the
compound includes five water molecules per formula unit of
CuSO 4 .
Everything you learned earlier in this chapter may be applied to
hydrates. The most important thing to remember when dealing with
hydrates is that the water molecules are treated as completely separate units—as if the water molecules were atoms of another element
in the compound. When giving the percent composition of hydrates,
the percent water is given separately, even if the compound contains
other oxygen or hydrogen atoms. The following example problem
shows you how to determine the number of water molecules
included in a hydrate.
Example Problem 10-10
Determining the Formula of a Hydrate
A hydrate of aluminum bromide is composed of 71.16% AlBr 3 and
28.84% H 2 O. What is the formula for the hydrate?
Again, when dealing with percent composition, assume you are
working with a 100-g sample of the compound. A 100-g sample of
this hydrate would consist of 71.16 g of AlBr 3 and 28.84 g of H 2 O.
By determining the ratio of moles of AlBr 3 to moles of H 2 O in this
sample, you can write the formula for the hydrate.
First, calculate the moles of AlBr 3 and the moles of H 2 O. Before
starting, you must add the molar masses of the atoms in AlBr 3 and
in H 2 O to determine the molar mass of each compound.
molar mass AlBr 3 ϭ 26.98 g ϩ 3(79.90 g) ϭ 266.68 g/mol
molar mass H 2 O ϭ 2(1.008 g) ϩ 15.999 g ϭ 18.015 g/mol
102 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
10.5 The Formula for a Hydrate
Many compounds, particularly ionic compounds, incorporate a specific number of water molecules into their crystals when they
crystallize from aqueous solution. Compounds that include water in
their crystal structures are called hydrates. The formula for a
hydrate is written by adding the formula for water to the formula
for the compound. The number of water molecules included per
formula unit is indicated by a coefficient. The formula for the blue
crystals of copper(II) sulfate is CuSO 4 и 5H 2 O, indicating that the
compound includes five water molecules per formula unit of
CuSO 4 .
Everything you learned earlier in this chapter may be applied to
hydrates. The most important thing to remember when dealing with
hydrates is that the water molecules are treated as completely separate units—as if the water molecules were atoms of another element
in the compound. When giving the percent composition of hydrates,
the percent water is given separately, even if the compound contains
other oxygen or hydrogen atoms. The following example problem
shows you how to determine the number of water molecules
included in a hydrate.
Example Problem 10-10
Determining the Formula of a Hydrate
A hydrate of aluminum bromide is composed of 71.16% AlBr 3 and
28.84% H 2 O. What is the formula for the hydrate?
Again, when dealing with percent composition, assume you are
working with a 100-g sample of the compound. A 100-g sample of
this hydrate would consist of 71.16 g of AlBr 3 and 28.84 g of H 2 O.
By determining the ratio of moles of AlBr 3 to moles of H 2 O in this
sample, you can write the formula for the hydrate.
First, calculate the moles of AlBr 3 and the moles of H 2 O. Before
starting, you must add the molar masses of the atoms in AlBr 3 and
in H 2 O to determine the molar mass of each compound.
molar mass AlBr 3 ϭ 26.98 g ϩ 3(79.90 g) ϭ 266.68 g/mol
molar mass H 2 O ϭ 2(1.008 g) ϩ 15.999 g ϭ 18.015 g/mol