Don’t forget to
hydrate!
Formula of a Hydrate
A hydrate is a pure substance that incorporates water molecules into its crystalline structure. The ratio
of moles of water to one mole of the compound is a small whole number. Calcium chloride dihydrate,
which is used as road salt, is an example of a hydrate with the chemical formula CaCl2·2H2O. The dot
indicates that there are 2 H2O molecules attached to each formula unit of CaCl2, and the mole ratio for
this compound is 2:1. By heating a hydrate, you can evaporate the water to yield an anhydrous salt. In
this lab, you will conduct an experiment to identify an unknown hydrate.
Objectives:
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Calculate the % H2O in an unknown hydrate
Determine the moles of H2O in the chemical formula for the hydrate
Identify the unknown compound
Materials:
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Ring stand and ring
Bunsen burner
Hot plate
Beaker
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Evaporating dish
Balance
Spatula
Unknown hydrate (2 grams)
Pre-lab:
Complete the following questions before the start of lab (use the back of this page for more space).
1. Describe the procedure you will follow to find the mass of water and the mass of anhydrous salt
contained in your unknown hydrate. Be specific about the equipment you will use, the steps you
will follow, and the timing of your experiment. You may find it helpful to draw your
experimental set-up.
2. Research the concept of “heating to constant mass”. How will you use this concept to know
when your experiment is finished?
3. Write an equation to calculate the %H2O in your unknown hydrate (Hint: think about how to
calculate % composition).
% H2O = --------------------------- x 100%
4. List 2 specific safety risks you will face during the experiment and explain what you will do to
reduce these risks.
Experiment:
Use the space below record your in-lab procedure and observations. Remember, your procedure should
include enough detail so that someone else could repeat your experiment exactly from what you write.
You can use up to 2 grams of unknown hydrate in your experiment. Let your sample cool before each
weighing.
Procedure:
Observations:
What data do you need to collect?
Unknown hydrate label
Mass of container
Clean-up: Dispose of your leftover anhydrous salt in the garbage. Clean all glassware thoroughly and
wash your hands.
Analysis:
Show all of your work in the space provided or on a separate sheet of paper. Remember to use correct
significant figures and labels.
1. Your unknown hydrate is one of the following substances. Complete the table to calculate the
theoretical %H2O for each hydrate.
Reference Table
CuSO4·5H2O
MgSO4·7H2O
ZnSO4·7H2O
Molar mass (anhydrous salt)
Molar mass (water)
Molar mass (whole hydrate)
% H2O in the hydrate
2. Calculate (a) the original mass of the hydrate, (b) the mass of water lost during heating, and (c)
the %H2O in the hydrate.
3. Compare your %H2O to the table of possible unknowns. What is the probable identity of your
unknown hydrate? How do you know?
4. Calculate the moles of anhydrous salt left at the end of your experiment.
5. Calculate the moles of water lost during heating.
6. Calculate the mole ratio of water to anhydrous salt for your hydrate. What is the complete
chemical formula of your unknown?
Error:
1. Calculate the percent error in your mole ratio using the formula below (the “actual” mole ratio
comes from the correct chemical formula, and the “experimental” ratio was calculated in #6).
%Error = (Actual – Experimental)/Actual x 100%
2. Consider the following potential sources of error in this experiment. Predict whether the
experimental %H2O in the hydrate will be too high (H), too low (L), or unchanged (NC) as a result
of each error. Explain your reasoning.
Source of Experimental Error:
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Water was observed around the edge of the container after heating.
Result
______
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The anhydrous residue decomposed upon heating, releasing a gas.
______
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The hydrate spattered during heating and some of the sample was lost.
______
3. What sources of error affected your experiment (if any)? If you were to do this experiment
again, what changes would you make to improve the procedure?
Conclusion
Complete the conclusion using complete sentences. Think about your claim, evidence, and reasoning
(use a separate sheet of paper if you need more space).
Claim: The identity of the unknown hydrate (go back to the purpose(s) of the lab)
Evidence: The data from your experiment that supports your claim and why it is relevant. Include
discussion of your percent error and what it means.
Reasoning/Explanation: The scientific reasoning that explains the concepts of the lab and that supports
your claim.