Pre-AP Lab – Percent Water in a Hydrate
Many crystalline compounds contain definite amounts of water, chemically combined
within their solid – state structure. Such compounds are called hydrates. In this
experiment you will analyze a hydrate to determine the percentage of water of hydration
present within it.
Pre-Lab Questions:
1. What does the word anhydrous mean? (A dictionary is as important to a chemist
as it is to any person who must communicate knowledge and understanding.)
2. What is the formula for Copper (II) sulfate?
3. What is the equation for calculating percent error?
4. What will happen if you add cold water to a piece of HOT glassware?
Procedure:
1. Weigh a clean dry evaporating dish to the maximum accuracy of your balance.
Record its weight on the report sheet.
2. Into the evaporating dish, weigh out between 2.50 and 3.50 grams of hydrated
copper (II) sulfate (also called cupric sulfate), the “blue vitriol” of alchemist. The
solid should first be ground in a mortar unless small granular samples are
available. Record the weight of the evaporating dish and its contents on the report
sheet.
3. Calculate the weight of the hydrate.
4. Place the evaporating dish and its contents on a hot plate. Turn the hot plate onto
at least 500C. Note and record any changes.
5. Observe the sample from time to time during heating. After approximately 10-15
minute of heating, allow the evaporating dish to cool to room temperature (This
will take approximately 10-15 minutes to cool)
6. Weigh the evaporating dish and solid residue to the maximum accuracy of your
balance. Record this weight on the report sheet.
7. The residue is anhydrous copper (II) sulfate. Calculate its weight.
8. Calculate the weight of water lost by your original sample.
9. Calculate the percentage of water of hydration in your sample.
10. After the evaporating dish has completely cooled, add a small amount of water to
the residue and record your observations.
11. Place residue that is left in the trash and clean your lab station.
Data/Observation Table:
Weight of empty evaporating dish
__________
Weight of evaporating dish & crystals before heating
__________
Weight of crystals before heating
__________
Observations at step 4:
__________________________________
__________________________________
Weight of evaporating dish and crystals after heating
__________
Weight of crystals after heating
__________
Observations at step 10
__________________________________
__________________________________
Post Lab Questions:
1. From the mass of the crystals before heating and after heating, determine the loss
or gain of mass.
2. The mass change is what percent of the original mass of hydrated crystals? This
is your experimental result.
3. Calculate the molecular weight of copper (II) sulfate.
4. Divide the grams of anhydrous copper (II) sulfate, by its molecular weight.
5. Divide the grams of water by its molecular weight.
6. Divide the answer to 5 and 6 by the smallest, in order to get the ratio of CuSO4 to
H2O. Write the empirical formula of your compound. (Round off to whole
numbers. Don’t worry about multiplying to get the whole number.)
______CuSO4 x ______ H2O
7. Assume that the correct formula is CuSO4 x 5H2O. Using this formula, calculate
the theoretical percentage of water in the hydrate.
8. Compare the experimental percentage of water in the original crystals with the
theoretical percentage and calculate the percent of error.
Difference in experimental value and theoretical value
Percent Error =
X 100
Theoretical Value
Lab Report
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3.
4.
Do not turn in this paper.
Write a lab report in your composition notebook using the rubric below.
Lab is due one week after completing the lab (Friday, 1/23/15).
Lab is worth 4 times a daily grade.
Lab Write Ups:
Hand-Written: Hand write the entire lab report in your lab notebook (5 points)
Purpose: Why are we doing the Lab? What is the objective? What do we hope to learn? (10 points)
Materials: What tools did you use, both chemical and laboratory instruments? Make a list. (10 points)
Procedure: What steps did you do to do the lab? Don’t copy the instructions. Summarize. Sometimes
we change the way you do the lab. Write it the way the lab was done, not the way it is written. (10 points)
Pre Lab Questions: Many labs have questions you have to answer before going to lab. Write the question
and the answer. (15 points)
Data Tables: Masses, volumes, observations. Put it in a table so it can be easily referenced. (15 points)
Post Lab Questions: Every lab has questions after the lab. Write the questions and answer them. If math
was involved, make sure that you show your work. (25 points)
Conclusion: What did you learn? What sources of experimental error were encountered? What can you
do better next time to get rid of those sources of error? (10 points)