Formula of a Hydrate
Introduction:
Many salts form compounds in which a definite number of moles of
water are combined with each mole of the anhydrous salt. Such
compounds are called hydrates. The water is an integral part of the
crystalline structure of the compound. When salt crystallizes from an
aqueous solution, the number of water molecules bound to the metal ion
are characteristic of the metal and are in definite proportion. In the
formula of a hydrate a dot is used to separate the formula of the
anhydrous salt from the number of molecules of water. For example, the
formula of calcium sulfate dihydrate is written as CaSO4 · 2 H2O not as
H4CaSO6.
Heat can transform a hydrate into an anhydrous salt. The water
can often be seen escaping as steam.
hydrated salt
anhydrous salt + water
This process is reversible; adding water to the anhydrous salt will
rehydrate the salt and regenerate the hydrate.
Some anhydrous salts are capable of becoming hydrated upon
exposure to the moisture in the air. These salts are called hygroscopic
salts and can be used as drying agents or desiccants. Some salts are able
to absorb so much moisture from the air that they can dissolve
themselves. Such compounds are said to be deliquescent.
Since many hydrates contain water in a stoichiometric quantity, it
is possible to determine the mole ratio of water to salt. First, you would
determine the weight of the water lost from the hydrate by heating a
weighed sample. From the weight of the water lost, you can calculate the
percent of water in the hydrate. From the weight of the water lost you
can also determine the number of water molecules in the hydrated salt
and so the mole ratio.
Procedure:
Properties of anhydrous CaCl2.
Take a small spatula full of anhydrous CaCl2 and place it on a
watch glass. Set the watch glass to the side, out of the way, and continue
the rest of the lab. From time to time during the lab period, look at the
solid and determine what happened to the solid.
2
Composition of a Hydrate.
Weigh a clean, dry crucible and cover to 0.001 grams. Record the
weight on the report sheet. Place between 3 and 4 grams of the hydrate
of copper(II) sulfate into the weighed crucible, and weigh the crucible,
cover, and hydrate. Record the weight on the report sheet.
Place the covered crucible and hydrate in the clay triangle. Move
the cover so that it is slightly ajar. See apparatus below:
Begin heating the crucible with a small flame for 5 minutes. If any
spattering of the solid occurs, remove the heat and completely cover the
crucible. Gradually increase the flame until the blue inner cone touches the
bottom of the crucible. Torch to red hot for an addition 12 minutes.
Remove the heat and using crucible tongs, place the covered crucible
on some wire gauze to cool to room temperature. Weigh the crucible, cover,
and contents. Record on report sheet.
Repeat the procedure of heating (about 6 minutes), cooling, and
weighing the crucible. The results of the weighings should agree within
0.05 grams. If the differences in weights is greater than 0.05 grams, repeat
heating, cooling, and weighing a third time.
Before you toss the white anhydrous salt, add a few drop of distilled
water to the salt. Describe what happens on the report form.
3
Name
Date
Report for Hydrate Experiment:
Data:
1. weight of crucible and cover
g
2. weight of crucible, cover and hydrate
g
3. weight of hydrate (#2 – #1)
g
4. weight of crucible, cover, and sample after
first heating
5. weight of crucible, cover, and sample after
second heating
6. weight of crucible, cover, and sample after
third heating (if necessary)
g
g
g
7. weight of anhydrous salt (#5 or #6 – #1)
g
8. weight of water lost (#3 – #7)
g
Calculations:
9.
Calculate the percent of water in the hydrate.
%=
10.
step # 8
× 100
step # 3
Find the moles of water lost.
moles of water lost =
step # 8
18.00
(mole weight of water)
4
11.
Calculate the moles of anhydrous CuSO4.
moles of anhydrous CuSO4 =
12.
159.6
(mole weight of CuSO4 )
Find the ratio of moles of water per mole of CuSO4.
ratio of water to hydrate =
13.
step # 7
step #10
step #11
Based on your calculations from step #12 above, what is the
formula of the hydrate of copper(II) sulfate?
CuSO4 · ? H2O
Name your hydrate:
14.
What happened when you added a few drops of distilled water to
the anhydride at the close of the lab?