Ch. 10: The Mole
Sec. 10.5: The Formula for a Hydrate
Formula for a Hydrate
• Determine the formula for a hydrate from
laboratory data.
Formula for a Hydrate
• A hydrate is a
compound that has a
specific number of
water molecules
bound to its atoms.
Formula for a Hydrate
• In the formula for a hydrate, the number of
water molecules associated with each
formula unit of the ionic compound is
written following a dot
Na2CO3•10H2O
• This is the same thing as saying that for
every 1 mole of Na2CO3, there are 10 moles
of water. There is a 1:10 mole ratio.
Formula for a Hydrate
• We know that to analyze the amount of
water in a hydrate you must drive off the
water of hydration.
• Often this is done by heating the compound.
• The substance remaining is anhydrous or
“without water”.
Formula for a Hydrate
• Suppose you have a
5.00 g sample of a
hydrate of cobalt
chloride.
• Its formula is
CoCl2•xH2O
• The value of x
indicates the number
of moles of water
associated with 1 mole
of cobalt chloride. X
must be determined.
Formula for a Hydrate
• The sample of the
hydrate is heated to
drive off the water of
hydration.
• The mass of the
anhydrous cobalt
chloride is determined.
It is 2.72 g.
Formula for a Hydrate
• The mass of the water of hydration is the
difference between the mass of the hydrate
(5.00 g) and the mass of the anhydrous
compound (2.80 g).
• 5.00 g – 2.72 g = 2.28 g H2O
Formula for a Hydrate
• This means there was 2.72 g of CoCl2 and
2.28 g of H2O in the hydrate.
• Those masses must now be converted to
moles.
– 2.72 g CoCl2 x 1 mole = 0.0209 mol CoCl2
129.9 g
– 2.28 g H2O x 1 mole = 0.127 mol H2O
18 g
Formula for a Hydrate
• Now the following formula must be used to
find x in CoCl2•xH2O
x = moles H2O/moles anhydrous compound
so, X = .127/.0209 = 6
• This means 6 moles of water are associated
with 1 mole of cobalt chloride.
• The formula is CoCl2 • 6H2O
• The name: cobalt(II) chloride hexahydrate.
Practice Problems
• 2.5 g of a copper sulfate hydrate
(CuSO4 • xH2O) is heated. After
heating, the sample is massed at 1.59
g. What is the formula and name of
this hydrate?
• A hydrate is found to have the following
percent composition: 48.8% MgSO4 and
51.2% H2O. What is the formula and
name for this hydrate?