Name: _____________________________
Determining the Formula Of A Hydrate Honors Chemistry
Background:
Hydrates are ionic compounds (salts) that take up specific amounts of water molecules as part of their
crystal structure. This water can be driven off by the application of heat. Since the ratio of the ionic
compound to water molecules is always the same, the percentage of water should be the same for each
sample.
One such compound that can incorporate water into its structure is Copper (II) sulfate. In most cases,
heating can eliminate the water in a hydrated molecule. Since ionic compounds have a high melting
point, the trapped liquid water is turned into gas and escapes the crystal.
Once the water is removed the remaining salt is called an anhydrous salt.
Some extra info:
Table salt usually contains a small amount of hydrated calcium chloride, CaCl2  H2O, and a hydrated
magnesium chloride, MgCl2  6H2O. On humid days these two impurities absorb moisture and cause the
table salt to become wet and to “cake”. To prevent this, some manufacturers add an anti-caking agent
such as magnesium silicate, Mg2Si3O8, which forms a hydrate that does not become wet.
Purpose:
In this lab, you will be given a sample of crystalline Copper (II) sulfate and will determine the percentage
of water in this sample. After determining the percent composition of water in a hydrate, you will
determine the empirical formula of the hydrate you are working with.
Helpful Information: Hydrates have a formula of Salt  n H2O , where n is the number of water molecules
attached to each salt molecule.
Materials:
Evaporating Dish
Watch glass
Ring Stand (with ring)
Clay Triangle
Copper (II) Sulfate
Pre-Lab Questions: Type your answers to these using complete sentences
1. What two things make up hydrates?
2. In order to determine the percent composition and the empirical formula of a hydrate, you must know
how much water is in the hydrate. How can you determine this?
Procedure:
1. Obtain a clean, completely dry evaporating dish and watch glass. Determine the mass of both. Record
this in your data table.
2. Fill the evaporating dish approximately half full with Copper (II) Sulfate crystals. Cover with the
watch glass and record the mass again. Remember to record proper number of significant figures.
3. Set up a ring stand with a clay triangle (see set-up at teacher’s lab bench). Place the evaporating dish
with watch glass cover (to prevent losing any material while heating) on the clay triangle and heat the
dish with a Bunsen burner until there is no blue color left showing. Be careful!! If you overheat, the
contents will spatter and can burn you!
4. Allow the dish and watch glass to cool completely. Then, take the final mass of the dish, chemical,
and watch glass.
5. Cleanup: Dump out chemical into waste beaker under hood. Carefully wash and dry evaporating dish
and watch glass. Return equipment to original location.
Calculations:
1. Determine the mass of the salt post-dehydration. Record this value (w/ proper significant figures) in
your data table.
2. Determine the mass of the water in the salt. Record this value (w/ proper significant figures) in your
data table.
3. Determine the percent composition of water from the original hydrate. Show your work (including
FORMULA and UNITS).
4. Determine the empirical formula of the hydrate. Show all work (including FORMULA and UNITS)
5. Determine your percent error if the accepted value is 36.08% water.
6. If the accepted value is 36.08% water, determine the proper empirical formula.
Conclusion: Type the answers to the following questions. You must answer in complete sentences, but
answers may be divided / separated by question.
1. If you stopped your experiment short, how would that affect the overall experiment? Would it affect
the empirical formula that you determined? How? Be specific.
2. Explain possible sources of error in your experiment (discuss at least 3).
3. What is a possible improvement that you could make to the procedure that would eliminate error in
your experiment? Discuss this improvement and explain how it would address areas of error.
4. A 5.0g sample of Cu(NO3)2  nH2O is heated. After the water has been driven off, 3.9 g of Cu(NO3)2
remain. What is the empirical formula of this hydrate? Show all work.