Lab - The Formula of a Hydrate 4351 (35 points)
Introduction: Many salts which have been crystallized from a water solution appear to be
perfectly dry, yet when heated yields large quantities of water. The crystals change form, even
color sometimes, as water is driven off. This suggests that water was present as part of the
crystal structure. Such compounds are called hydrates. The number of moles of water present
per mole of anhydrous salt is usually some simple number. One example of a hydrate is cobalt
(II) chloride hexahydrate. Its pink crystals look and feel dry, yet each mole of the hydrate
contains six moles of water. Its formula is CoCl2 • 6H2O. The dot between CoCl2 and 6H2O
does not mean multiplication.
It means that water molecules are rather loosely attached to the other atoms. The molecular
mass of CoCl2 • 6H2O is: 58.9 + 2(35.45) + 6(18.0) = 237.8 grams/moles.
The hydrate is heated in a crucible. For accurate results, the crucible is handled with tongs
because fingerprints can add weighable mass to the crucible. Porcelain crucibles absorb a bit of
weighable moisture from the air. For this reason, the porcelain crucible and lid is also pre-fired
(pre-heating to high temperature) to constant mass before the pre-weighing. This determines the
mass of the completely dry crucible and lid. At least two firings, coolings, and weighings resulting
in exactly the same mass are needed to confirm constant (completely dry) mass of the crucible
and lid and similarly again for the crucible, lid, and sample residue inside. This is called massing
to constant weight.
A desiccator is a container which contains a desiccant to absorb moisture from the air inside, so
the air inside will be completely dry. The desiccant is usually a anhydrous salt, which combines
with the water from the air to form a stable hydrate. Refer to the figure on the attached page.
Prelab
1. Read the entire lab
2. Summarize the procedure in your lab notebook
3. Write the objectives for this lab!
4. List all materials and equipment.
5. Prepare an appropriate data table
6. Answer pre-lab questions.
Pre-lab Questions
In this experiment you will be given cupric sulfate hydrate. You will determine the: a) mass of
water driven off by heating. b) amount of anhydrous salt, which remains.
1. Find the mass of 1 mole of the anhydrous salt, cupric sulfate. Show work and units!
2. Experimentally, how would you obtain the mass of the anhydrous salt CuSO4? Explain.
3. Experimentally, how would you obtain the mass of water? Explain.
4. How would you convert the masses of H2O and CuSO4 to moles?
5. How is the formula for the hydrate obtained from the moles of CuSO4 and H2O?
6. What is “massing to constant weight”?
Procedure
1. Wear safety glasses and apron.
2. Place a clean, dry crucible with cover in a triangle mounted on an iron ring. Leave the cover
askew so that any water in the crucible can easily escape. Heat the empty crucible with a high
Bunsen burner flame for two or three minutes to clean and dry the crucible.
3. Do not touch the clean crucible, nor should you weight it while it is still hot. Use tongs to
transfer the crucible to the counter to cool. After about five minutes, use tongs to transport the
crucible and its cover to a balance and find the mass.
4. Put enough of the hydrate crystals in the crucible to fill it one-fourth to one-third full.
(Less than 5 g). Weigh the crucible with cover and content.
Record the masses and write your observations of the hydrate in your data table.
5. Place the crucible, with its cover only slightly askew, on the triangle. See figure on attached
page. Begin heating gently so as to avoid splattering. Gradually increase the flame until the
crucible bottom is at most a dull red.
Maintain this high temperature for five minutes. Use tongs to lift the cover and observe the color of
the salt. If no more blue hydrate is present, use tongs to transfer the hot crucible
and cover to a dessicator. Be careful! The crucible and its cover are VERY HOT!
6. Wait about five minutes, until the crucible is cool enough to touch. Before touching, put your
hand close to the crucible and feel the heat radiating from it. If it is still hot don’t touch it. When
cool transfer the crucible and its cover to the balance and weigh them. Record your data and
observations in your data table.
7. To make sure all of the water is driven off heat the crucible and its cover to dull redness again.
Carefully transfer them to a dessicator. When cool, weigh again. If your results do not agree
within 0.03 grams from the previous evaporation, you will need to repeat the heating and cooling
process until it does.
8. Before you dispose of the anhydrous salt, cupric sulfate add a few drops of water to
the crucible and record you observations.
9. Clean up. Dispose of the salt in the trash. Wash the crucible well with soap and water. Return
all equipment to the proper bins. Wash your hands.
Calculations- Show your work and units! Use correct Significant Figures!
Write a short explanation for each.
1. Calculate the number of moles of the anhydrous salt you prepared.
2. Calculate the number of moles of water driven off from your sample.
3. Calculate the whole number mole ratio of water to anhydrous salt.
4. Write the empirical formula for the hydrate, CuSO4•xH2O.
5. Based on your data of the mass of hydrate and the mass of water evaporated, calculate the %
composition of water in the sample of hydrate you used. Calculate the % composition of water in the
hydrate based on its theoretical empirical formula.
6. Calculate the % error of the water mass you measured based on the calculations in question #5
by using the formula below.
% error 
Theoretical  Experimental
x 100
Theoretical
Analysis Questions
1. Why is the formula of the water in a hydrated compound separated by a dot
(e.g. BaCl2•2H2O) instead of being included in one formula (BaCl2H4O2)? How did this experiment
prove this?
2. Why was the crucible placed in the dessicator?
3. Why is the hydrate heated at least twice?
4. Suggest reasons why the procedure used in the experiment might not be suitable for
all hydrates?
5. Many different drying agents could have been used in your dessicator. Give at least three
characteristics this desiccant should have.
6. Is the process of removing water from a hydrate endothermic or exothermic? What about the
reverse reaction (of adding water to the anhydrous salt)? Explain.
7. Why did the anhydrous copper sulfate change color when water was added?
8. List three possible sources of error in this experiment, and state whether each would have
caused a high or low value for x in the reported formula.
Write a conclusion paragraph.