THE FORMULA OF A HYDRATE
any salts crystallized from water solutions appear to be perfectly dry;
yet when heated, they liberate large quantities of water. The crystals change
form, even color, as the water is driven off. Such compounds are called
hydrates. The number of moles of water present per mole of anhydrous salt
(salt minus water of crystallization) is usually a whole number. One
example is the hydrate of copper sulfate. Its blue crystals look and feel dry.
Yet, each mole of hydrate contains 5 moles of water. Its formula is
CuSO4.5H2O. The dot between the CuSO4 and the 5H2O does not mean
multiplication. It indicates that 5 water molecules are bound to the other
atoms as ligands. The molar mass of CuSO4.5H2O is
63.5 + 32.1 + 64.0 + 5(18.0) = 249.6 g
In this experiment, you will be given appropriate hydrate(s) selected by
your teacher. You will determine the mass of the hydrate, the mass of water
driven off by heating the hydrate, and the mass of the anhydrous salt that
remains. By calculating the number of moles of water driven off and the
number of moles of anhydrous salt remaining, you will be able to find the
empirical formula of the hydrate.
MATERIALS
5.25" glass pasteur pipets (6)
ring stand, wire gauze, iron ring, and bunsen burner
samples of various solids
wood splint spatula for transferring solid to pipets
PROCEDURE
Caution: Put on your goggles and apron now!!
1. Obtain and mass a dry pasteur pipet. Record the data in the data table.
Zero the balance each time and use the same balance throughout this
experiment.
2. Fill the pipet with approximately 1/2 gram of an unknown hydrate.
This can be accomplished by trial and error. Try to avoid packing the
crystals in too tightly.
3. Mass the pipet again and record the mass on the table.
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4. Lay the pipet on a square of wire gauze that does not have a ceramic
center. If you can’t find one without a ceramic center, then lay the
pipet along one edge so that the flame from the bunsen burner can heat
the entire length of the pipet. Place the wire gauze and pipet on an iron
ring and ring stand. Make sure to distribute the hydrate along the
length of the pipet.
5. Heat the pipet by gently waving a bunsen burner flame back and forth
underneath the wire gauze. Write down any observations that may
occur as you heat the hydrate.
6. Heat for approximately 5 minutes. Allow the pipet to cool before
handling.
WARNING: Hot glass looks exactly like cold glass. Avoid contact.
7. Mass the pipet and record this information on the data table.
8. Heat again for 5 minutes. Repeat steps #5 through #7 until the mass
the of the hydrate and pipet is constant.
9. Repeat steps #1 through #8 with two more samples of the same
hydrate.
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DATA TABLE
Trial #1
Trial #2
Trial #3
mass of empty pipet (g)
mass of hydrate & pipet (g)
mass after 1st heating (g)
mass after 2nd heating (g)
mass after 3rd heating (g)
constant mass of anhydrous
salt & pipet (g)
mass of water driven off (g)
#moles of water
formula weight of anhydrous
salt (g)
grams of anhydrous salt (g)
moles of anhydrous salt
#moles of water/#moles of
anhydrous salt
empirical formula of hydrate
experimental
empirical formula of hydrate
theoretical
QUESTIONS
1. Can you suggest reasons why the procedure used in this experiment
might not be suitable for all hydrates?
2. Ask your teacher for the formula(e) of the anhydrous salt(s). Calculate
your empirical formula(e) of the hydrated salt(s).
3. Ask your teacher for the correct formula(e) of the hydrated salt(s). Did
your experimental data give the correct results? If not, why do you
think they were different?
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