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C-7
After obtaining this mass measurement, you must reheat the test tube and its contents a second
time, allow them to cool, and record the mass again to determine if a constant mass (mass
measurements within 1% after heating) for the anhydrous sample has been reached. This time, do
not heat as strongly or as long as before. If decomposition occurs, stop heating immediately. Once
the anhydrous mass is constant (1%), the dehydration process is complete.
NOTE: It may be necessary to reheat the sample 3 or 4 times until a constant mass for the
anhydrous salt is obtained.
E.
On your Observations Sheet, calculate the mass percent of water in your unknown hydrate.
(Remember to use the correct number of significant figures.)
NOTE:
When you have finished this Part of the experiment, put the solid in the solid waste container
provided in the fumehood.
Part II. Properties of Some Hydrates
A.
Copper(II) Sulfate Pentahydrate
1.
Obtain a few crystals of copper(II) sulfate pentahydrate in a clean, dry test tube. Gently heat the
crystals by holding the tube, with a test tube clamp, in an inclined position over a Bunsen burner
flame. Do not point the open end of a test tube towards anyone. What evidence is there that water
was present in the crystals? Record your observations.
2.
Continue heating until no further change is observed. How does the residue differ in appearance
and composition, from the original substance?
3.
Allow the tube to cool, then add one to two drops of water to the residue. Record your
observations. Is the reaction reversible?
