Formula of a Hydrate
What is a 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 sulfate pentahydrate
What is an anhydrate?
•
The substance that remains after the water is
removed from a hydrate
•
Example: CuSo4
Anhydrous Copper 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. Identify what’s given:
A. Hydrate/anhydrate (go to step 2)
B. Anhydrate/water (skip to step 3)
2. If given hydrate and anhydrate, subtract to get water
3. Convert mass of anhydrate and water to moles of each,
respectively
4. Find the ratio of water to anhydrate (in moles)
5. Write the formula; ionic compound followed by ratio of
water
6. Name the compound
Example 1
A calcium chloride hydrate has a mass of 4.72g. After
heater 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
Step 5:
CaCl2 2H2O
Step 6:
calcium chloride dihydrate
Example 2
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?
Step 2:
15.67g hydrate
− 7.58g anhydrate
8.09g H2O
Magnesium carbonate:
Mg2+ CO32−
MgCO3
Step 3: 3.56g MgCO3 1mol MgCO3 = 0.0899 molMgCO
3
84.32g MgCO3
8.09g H2O 1mol H2O = 0.449 mol H2O
18.02g H2O
SIG FIGS IN ANSWER MUST MATCH GIVEN!!
Step 4: Ratio
0.449 mol H2O
0.0899 mol CaCl2
≈5
Step 5:
MgCO3 5H2O
Step 6:
magnesium carbonate pentahydrate
Example 3
What is the formula an name for a hydrate which
consists of 76.9% CaSO3 and 23.1% H2O?
Step 3: 76.9g CaSO3 1mol CaSO3
= 0.640 mol CaSO3
120.15g CaSO3
23.1g H2O 1mol H2O
18.02g H2O
= 1.28 mol H2O
SIG FIGS IN ANSWER MUST MATCH GIVEN!!
Step 4: Ratio
1.28 mol H2O
0.640 mol CaSO3
Step 5:
CaSO3 2H2O
Step 6:
calcium sulfite dihydrate
≈2
Example 4
What is the formula an name for a hydrate which
consists of 51.77% NiSO4 and 48.23% H2O?
Step 3: 51.77g NiSO4 1mol NiSO4
= 0.3345 mol NiSO4
154.67g NiSO4
48.23g H2O 1mol H2O
18.02g H2O
= 2.676 mol H2O
SIG FIGS IN ANSWER MUST MATCH GIVEN!!
Step 4: Ratio
2.676 mol H2O
0.3345 mol CaSO3
≈8
Step 5:
NiSO4 8H2O
Step 6:
nickel sulfate octahydrate