EMPIRICAL FORMULA OF
A HYDRATE
Many 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 (II) sulfate. Its blue crystals look and feel
dry. Yet each mole of hydrate contains 5 moles of water. Its
formula is CuSO4 . 5 H2O. The dot between the CuSO4
and the 5 H2O does NOT mean multiplication. It indicates that
5 water molecules are bound to the other atoms. The molar mass of CuSO4 . 5 H2O is 249.6
g/mole.
In this experiment, you
will determine the empirical formula of a hydrate. You will do so by
heating the hydrate to drive off the water. You will measure the mass of
the water driven off the hydrate by heating it and you will measure 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
3 - 5.25"
glass test tubes
ring stand, wire gauze, iron ring, & burner
hydrated salt
wood splint spatula for transferring solid to test tubes
Procedure
*PUT YOUR GOGGLES & APRON ON NOW!*
1.
Obtain and label three dry test tubes #1, #2, and #3. Weigh the test
tubes to the nearest 0.01 g. Record these data on the data table
below. Remember to zero the balance each time you use it. Also
remember to use the same balance throughout the experiment.
2. Add 0.5 - 0.8 g
of the hydrate to each of the test tubes. Do not pack the crystals.
3. Reweigh the three
test tubes and record the masses, again to the nearest 0.01 g.
4. Lay the test
tubes on the square wire gauze. Turn the corners of the wire gauze up
slightly so the test tubes do not roll off. You must be able to heat the
entire length of the test tube with the burner to drive off all the water of
hydration. Place the wire gauze and the three test tubes on the iron ring
and heat the three samples gently and evenly along their entire lengths.
5. Heat the test
tubes by gently waving the burner flame back and forth underneath the wire
gauze. Record any observations you make during the heating process.
6. Heat for
approximately 5 minutes. Allow the test tubes to cool before
handling. This may take several minutes. (Remember that the glass
is very hot even though it may appear to be cool.)
7. Weigh the three
samples and record the data.
8. Reheat the three
samples for an additional five minutes, cool, and reweigh until the mass of the
anhydrous salt and test tube is constant.
What does “anhydrous” mean?
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Trial 1 |
Trial 2 |
Trial 3 |
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1 |
mass of
empty test tube (g) |
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2 |
mass of
hydrate & test tube (g) |
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3 |
mass
after 1st heating (g) |
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4 |
mass
after 2nd heating (g) |
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5 |
constant
mass of anhydrous salt & test tube (final mass, g) |
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6 |
mass of
water driven off (g) |
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7 |
# of
moles of water |
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8 |
molar
mass of anhydrous salt (______________) |
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9 |
grams of
anhydrous salt (g) |
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10 |
moles of
anhydrous salt |
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11 |
mole
ratio of H2O to anhydrous salt |
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12 |
Empirical
formula of hydrate - |
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13 |
Empirical
formula of hydrate - |
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Calculations
Show all
calculations necessary to calculate the empirical formula for the hydrated
salt. You have to show your work for only one trial.
Grading
To receive full
credit, the data table must be filled in completely and correctly for all three
trials. You must include a sample calculation sheet along with your data
table.