Formula of a Hydrate
HYDRATE
•
An ionic compound that contains water
molecules in its structure
•
The formula states how many mol of water
molecules per one mol of ionic compound
•
Example: CuSo4  5 H20
Copper(II) sulfate pentahydrate
ANHYDRATE
•
The substance that remains after the water is
removed from a hydrate
•
Example: CuSo4
Anhydrous Copper(II) sulfate
Prefixes
1
mono
2
di
3
tri
4
tetra
5
penta
6
hexa
7
hepta
8
octa
9
nona
10
deca
Steps to find formula
1. Find the mass of anhydrate and water
2. Convert mass of both anhydrate and water to
moles, respectively
3. Find the mole ratio of water to anhydrate
4. Write the formula and name
Example i
A calcium chloride hydrate has a mass of 4.72g. After
heated for several minutes the mass of the anhydrate is
found to be 3.56g. Determine the formula and name of
the hydrate.
2+ Cl−
Calcium
chloride:
Ca
Step 2:
4.72g hydrate
− 3.56g anhydrate
CaCl2
1.16g H2O
Step 3:
3.56g CaCl2 1mol CaCl2 = 0.0321 molCaCl
2
110.98g CaCl2
1.16g H2O 1mol H2O = 0.0644 mol H2O
18.02g H2O
SIG FIGS IN ANSWER MUST MATCH GIVEN!!
Step 4: Ratio
0.0644 mol H2O
0.0321 mol CaCl2
≈ 2
1
Step 5:
CaCl2 2H2O
Step 6:
calcium chloride dihydrate
Example ii
A 15.67 g sample of a hydrate of magnesium carbonate
was heated, without decomposing the carbonate, to
drive off the water. The mass of the anhydrate was found
to be 7.58 g. What is the formula and name of the
hydrate?
Example iii
What is the formula and name for a hydrate which
consists of 76.9% CaSO3 and 23.1% H2O?