IGCSE Chemistry
Finding
the formula of a hydrated salt
The
purpose of this experiment is to determine the empirical formula of a hydrate.
Hydrates
are inorganic salts that contain a specific number of water molecules loosely
attached.
An
example is sodium carbonate decahydrate (washing soda). The formula for this
substance is Na2CO3•10H2O. The raised dot does not mean “multiply”; rather, it means that
the water molecules are loosely attached.
Hydrates
can normally be decomposed into the anhydrous (“without water”) salt and water
by gentle heating according to the equation below. From the data collected the
number of molecules of hydrated water will be determined per formula unit of
anhydrous salt.
CuSO4•xH2O ® CuSO4 + xH2O
What to do
1. Determine the mass of an
evaporating dish and record it to the nearest 0.01 g.
2. Add 2.00 g of copper sulphate hydrate to the
evaporating dish and record the combined mass of the
salt and evaporating dish.
3. Place the evaporating
dish on the wire gauze on your tripod. Begin heating the sample slowly.
4. After about 10 minutes of gentle heating,
increase the heat until no further change in the salt is
apparent. At this point, the salt will seem to have
changed from a crystalline solid to a grainy solid.
You may need to use your glass stir rod to
gently break up some of the crystals to assist in the
evaporation of the
water.
5. Use tongs to place the evaporating dish on a
heat proof mat in order to cool. Cover the salt with a
watch glass to slow
the possible re-absorption of water by the anhydrous salt.
6. When the evaporating dish has cooled sufficiently
to be handled, determine the mass of the
evaporating dish and
contents.
7. Return
the evaporating dish to the ring stand and re-heat it. Allow it to cool, then
determine the mass
of the evaporating
dish with its contents again.
8. Continue this process
until a constant mass is attained.
9. Record the lowest mass of the crucible and
anhydrous salt.
Results
Design your own results table and
record the raw data you have obtained.
Also record any observations you have made.
Calculations
1. What is the mass of
hydrated copper sulphate used in this experiment?
2. What is
the mass of anhydrous copper sulphate left at the end of the experiment?
3. What is
the mass of water contained in the hydrated copper sulphate?
4. How many
moles of anhydrous copper sulphate are left?
5. How many
moles of water were in the hydrated copper sulphate?
6. What is
the simplest whole number ratio of moles of anhydrous copper sulphate to water?
7. What is the
formula of hydrated copper sulphate?
Questions
1. How do you
know when the reaction is complete?
2. How could you make this experiment more
accurate?
3. Give two
specific safety measures when carrying out this experiment?
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