1. The Percentage of Water in a Hydrate
Mole Theory Unit
Chemistry I Honors

2. A Review of Percent Composition
Remember that the percentage of any defined part of a system is defined as the total of that part divided by the total of all of the parts. The result of that division is then multiplied by 100 to give the final answer.
The percentage of “green” will be calculated as:
3 green
100 X
16 total
= %

3. Now applying this to chemical compounds…
Example: What is the percentage of oxygen in the compound sodium sulfate?
Sodium sulfate has the formula
Na2SO4
4 ( 16 ) grams O
[ 2(23) + 1(32.1) + 4(16) ] g Na2SO4 64 =
100 X
The percentage of oxygen will be calculated as:
142.1
45.04 % oxygen =

4. Now for:
Hydrates
cool compounds!!!
good for lab !!!

5. What is a Hydrate ???
These are compounds that consist of a regular ionic compound with some number of water molecules attached to the perimeter of that ionic molecule.
H2O
H2O
H2O
Ionic molecule
H2O
H2O

6. AX • n H2O Represents the ionic molecule
We can describe the compound as having a “generic formula” with the format:
The H2O is called
the
“water of
hydration”
AX • n H2O
Represents the
ionic molecule
a coefficient
The “dot” means
“attached to”

7. Naming these hydrates:
There will be two components to the name and the name will end up having three words:
Name the ionic molecule following the rules for any other ionic compound.
Write a covalent prefix for the number in the coefficient spot and attach the word “hydrate”.
Example: The compound has the formula MgCl2 • 6 H2O
Its name is magnesium chloride hexahydrate

8. Formula Weight of Hydrates:
You will still determine the formula weight using the formula
f.w. = Σ (inventory x atomic weight)
The only difference is that you will have to include the mass of the water molecules in the total.
It will help to remember that water has a
formula weight = 18 grams/mole

9. Example: Determine the formula weight of the compound: Na3PO4 • 4 H2O
Solving:
fw = 3(23) + 1(31) + 4(16) + 4(18)
= 236 grams/mole
Note that in this case the “dot” means “attached to” so we will ADD the mass of the water molecules. (we will not multiply like in math class)

10. Now for Percentage of Water
You can see that the water molecules make up an “identifiable part” of the complete compound.
Therefore, you can determine the percentage of water in a given hydrate.
It will be calculated as:
% water =
Total mass of water
X 100
Total mass of hydrate

11. Calculate the percentage of water in iron (III) chloride trihydrate
An example…
Calculate the percentage of water in iron (III) chloride trihydrate
Step #1 – write the formula
FeCl3 • 3 H2O
Step #2 – set up the calculation
3(18)
% H2O =
X 100
1(55.8) + 3(35.5) + 3(18) 54 =
X 100
= %
216.3