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Laboratory - The Formula of a Hydrate
INTRODUCTION Many ionic crystals, although dry to the touch, will
yield large quantities of water when heated. Salts that contain water as part of
their crystal structure are called hydrates. When the water is removed, the salt
is said to be in its anhydrous form. The term anhydrous means "without
water".
In this experiment you will determine the formula of the hydrate of
copper(II) sulphate. By heating the hydrate and driving off the water, you will
be able to determine the mass of water lost and the mass of anhydrous
copper(II) sulphate remaining. This will allow you to determine the number of
moles of water combined with each mole of copper (II) sulphate.
CuSO4 xH2O ---------- CuSO4 + xH2O
There should be a dot in the formula on the left hand side of the equation to
indicate that the water molecules are strongly attracted to the copper (II)
sulphate
PROBLEM What is the formula of the hydrate of copper (II) sulphate?
APPARATUS
MATERIALS
Bunsen burner
crucible and cover
iron ring
clay triangle
centigram balance
crucible tongs
ring stand
13 x 100 mm test tube
hydrated copper (II) sulphate
PROCEDURE
1. 1. Make a copy of the following data table to record your observations
and calculations.
Mass of crucible and cover
Mass of crucible, cover and CuSO4 xH2O
Mass of crucible, cover and CuSO4
Mass of CuSO4 xH2O
Mass of CuSO4
Mass of water lost
Amount of CuSO4
Amount of water
Amount of water per mole of CuSO4
1. 2. Assemble the apparatus.
2. 3. Heat a clean, dry crucible and cover for about 2 minutes. Allow the
3.
4.
5.
6.
7.
crucible and cover to cool for about 3 minutes and determine their mass
to the nearest 0.01 g.
4. Add approximately 3 g of copper (II) sulphate to the crucible. Find
the mass of the crucible, cover, and hydrate to the nearest 0.01 g.
5. Heat the crucible gently, with the cover slightly ajar, for about 5
minutes. Carefully remove the cover and examine the contents. If all
the blue colour has not yet disappeared, continue to heat the crucible
gently, examining the contents periodically, until all the blue colour has
disappeared.
6. When all the blue colour has disappeared, increase the flame
temperature until the bottom of the crucible is a dull red colour and
continue heating for 2 - 3 minutes.
7. Allow the covered crucible to cool and determine the final mass.
8. Place a small amount of the anhydrous copper (II) sulphate in a test
tube and add a few drops of water. Observe what happens. Feel the
bottom of the test tube.
CALCULATIONS
i) Using the information recorded in your data table calculate:



the mass of the hydrate, CuSO4 xH2O
the mass of the anhydrous copper (II) sulphate, CuSO4
the mass of the water lost
ii) From the mass of the water lost, determine the amount (in moles) of water
lost.
iii) Calculate the amount of anhydrous copper (II) sulphate remaining.
iv) How many moles of water were combined with 1 mol of anhydrous copper
(II) sulphate? Since it is only possible to have a whole number of water
molecules, round you answer off to the nearest integer.
QUESTIONS
a) Why was it necessary to heat the crucible and cover before determining the
initial mass?
b) Why must the crucible and cover be cooled before their mass is found?
c) How do you know when the copper (II) sulphate has been converted to its
anhydrous form?
d) What was the role of the crucible cover in this experiment?
e) Rewrite the equation for this reaction, replacing x with you experimental
value.
f) Does you experimental value for x agree with the accepted value? Account
for possible sources of experimental error.
g) What did you observe when water was added to the anhydrous salt?
Account for this observation. Was this an endothermic or exothermic reaction?
Reference: Percival Stan, Ross Wilson, Chemistry A Human Venture
Laboratory Manual, Irwin Publishing, Canada, 1988.
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