Transcript The Percentage of Water in a Hydrate

The Percentage of Water
in a Hydrate
Mole Theory Unit
Chemistry I Honors
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
X 100
16 total
= 18.75 %
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
=
The percentage of
oxygen will be
calculated as:
64
X
100
142.1
=
45.04 % oxygen
Now for:
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
We can describe the compound as having a
“generic formula” with the format:
AX • n H2O
Represents the
ionic molecule
a coefficient
The “dot” means
“attached to”
The H2O is called
the
“water of
hydration”
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
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
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)
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
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
% H2O =
=
3(18)
1(55.8) + 3(35.5) + 3(18)
54
216.3
X 100
X 100
= 24.97 %