Chapter 10: Section 5
Hydrates
Hydrate: a compound that has a specific number of water
molecules bound to its atoms.
Formula for a Hydrate: The number of water molecules
associated with each formula unit of the compound
Example: Na2CO3 s 10H2O
Called: sodium carbonate decahydrate
deca- means 10 and hydrate means water
Examples of hydrates
Analyzing a Hydrate
Must drive off the water by heating the compound
Substance remaining after heating is called the
Anhydrous: a compound “without water”.
Example: Na2CO3 s 10H2O
When heated water is driven off
Na2CO3 remains… this is the anhydrous
Determining the Formula for a Hydrate
Determine the # of moles of water associated with one mole
of the anhydrous.
HOW:
Convert the grams of water and anhydrous to moles…
divide by the smallest # of moles…
And that will give you the moles of water per anhydrous
Example Problem
A mass of 2.5 g of blue, hydrated copper sulfate is placed
in a crucible and heated. After eating, 1.59 g white
anhydrous copper sulfate remains. What is the formula for
the hydrate. Name the Hydrate.
FIRST:
Get the number of grams of the salt and water.
SECOND:
Find the # of moles of each (Molar Mass)
Third:
Divide by the smallest mole.
Thats the ratio… always 1 salt.
Example Problem
A mass of 2.5 g of blue, hydrated copper sulfate is placed
in a crucible and heated. After eating, 1.59 g white
anhydrous copper sulfate remains. What is the formula for
the hydrate. Name the Hydrate.
Known:
Mass of hydrate = 2.50 g CuSO4
Mass of anhydrous = 1.59 g CuSO4
Molar mass = 18.02 g/mol H20
Molar mass = 159.6 g/mol CuSO4
Unknown
Mass of water
Formula of Hydrate
Name of Hydrate
Example Problem
A mass of 2.5 g of blue, hydrated copper sulfate is placed
in a crucible and heated. After eating, 1.59 g white
anhydrous copper sulfate remains. What is the formula for
the hydrate. Name the Hydrate.
Mass of Water:
Mass of hydrate - Mass of anhydrous
2.50 g CuSO4 s
- 1.59 g CuSO4
0.91 g H20
x
H2O
Example Problem
A mass of 2.5 g of blue, hydrated copper sulfate is placed
in a crucible and heated. After eating, 1.59 g white
anhydrous copper sulfate remains. What is the formula for
the hydrate. Name the Hydrate.
Determine the # of moles of Water and anhydrous
Use the Molar mass of each
1.59 g CuSO4 1 mol CuSO4
= 0.00996 mol CuSO4
159.6 g CuSO4
0.91 g H20 1 mol H20
18.02 g H20
= 0.050 mol H20
Example Problem
A mass of 2.5 g of blue, hydrated copper sulfate is placed
in a crucible and heated. After eating, 1.59 g white
anhydrous copper sulfate remains. What is the formula for
the hydrate. Name the Hydrate.
Determine the mole ratio (x) of the anhydrous and water
Divide the # mols of anhydrous into the # mols of water
CuSO4 s
x
H2O
0.050 mol H20
= 5
0.00996 mol CuSO4
CuSO4 s
Name the Hydrate:
Copper (II) Sulfate Pentahydrate
5
H2O