Formula of a Hydrate
Name_________________________
Introduction
Many ionic compounds, when crystallized from water solution, take up definite
proportions of water as an integral part of their crystal structures. This water of
crystallization may be driven off by the application of heat. The number of moles of
water of crystallization driven off per mole of the anhydrous compound is a simple,
whole number. If the formula of the anhydrous compound is known, you can then
determine the formula of the hydrate.
Purpose
To determine the formula of the hydrate CuSO4  nH2O.
Safety Notes:
 Wear safety glasses throughout this lab.
 Tie back hair and remove jackets with long sleeves.
 Do not eat or drink in the lab; copper(II) sulfate has an LD50 of 300 mg.
Procedure:
1) Determine the mass of a clean, dry test tube to the nearest 0.001 gram, and record this
mass in the data table. Don’t worry if your test tube has an orange tint on the bottom
– there may be a small amount of copper than has become embedded in the glass. It
won’t affect your results.
2) Grind up some of the hydrated crystal using a mortar and pestle, and place about 2 cm
of the hydrated crystal in the bottom of the test tube with a scupula.
3) Determine the mass of the test tube and hydrated crystals to the nearest 0.001 g, and
record this mass in the data table. Make sure to use the same balance you used
previously.
4) Place the test tube on the ringstand with the test tube clamp. Make sure the mouth of
the test tube is pointing away from other people. Heat the test tube for about 10
minutes, or until the crystals have turned completely white, and no more water vapor
is seen escaping. As water droplets re-condense around the mouth of the test tube,
grab the burner by the base and heat away these water droplets.
5) Turn off the Bunsen burner and allow the test tube and contents to cool.
6) Determine the mass of the test tube plus the anhydrous crystal to the nearest 0.001 g,
and record this mass in the data table. Make sure to use the same balance you have
been using.
7) Clean out the test tube with a test tube brush, rinse it with distilled water, and place it
on the drying rack to dry.
Data Table:
Mass of Test Tube
Mass of Test Tube + CuSO4nH2O
Mass of Test Tube + CuSO4
Calculations: (Note: pay attention to significant figures throughout these calculations.)
1) Calculate the mass of the anhydrous copper(II) sulfate that remained after heating.
2) Calculate the mass of the water driven off.
3) Calculate the moles of the anhydrous copper(II) sulfate that remained after heating.
The formula for anhydrous copper(II) sulfate is CuSO4.
4) Calculate the moles of water driven off.
5) Determine the mole ratio of water to anhydrous compound by dividing your answer in
#4 by your answer in #3, paying attention to significant figures.
6) The correct, accepted value for n is 5; copper(II) sulfate exists in the hydrated form as
copper(II) sulfate pentahydrate, CuSO4  5H2O. Assuming the number “5” in this
formula has an infinite number of significant figures, calculate your percent error for
your answer obtained above.
% Error = │experimental value - accepted value│ X 100%
accepted value
Follow-up Question:
Suppose a student makes the mistake of not heating the test tube long enough. What
effect will this error have on the calculated value of “n”? Explain.
Pre-Lab: Must be complete before beginning the lab!
1) How can the water of crystallization be removed from an ionic compound?
2) What does the term “LD50” refer to?
3) What two pieces of equipment is used to grind up the compound in this lab?
4) How long should the compound in the test tube be heated? What physical change will
occur in the compound while it is being heated?
5) What should be done if water droplets form around the mouth of the test tube?
6) Why is it important to use the same balance to record the mass in steps 1, 3 and 6 of
the procedure?
7) Suppose a student does an experiment to determine the formula of the hydrate:
Cr(NO3)3  nH2O
The following data are obtained:
Mass of Test Tube
16.114 g
Mass of Test Tube + Cr(NO3)3  nH2O
34.909 g
Mass of Test Tube + Cr(NO3)3
27.335 g
Use this data to determine the value for n. (Answer: n = 8.916)
8) Suppose a student does an experiment to determine the formula of the hydrate:
FeCl2  nH2O
The following data are obtained:
Mass of Test Tube
20.480 g
Mass of Test Tube + FeCl2  nH2O
38.278 g
Mass of Test Tube + FeCl2
31.775 g
Use this data to determine the value for n. (Answer: n = 4.050)
Hypothetical Data:
Mass of Test Tube
22.960 g
Mass of Test Tube + CuSO4nH2O
28.274 g
Mass of Test Tube + CuSO4
26.334 g
Hypothetical Data:
Mass of Test Tube
22.960 g
Mass of Test Tube + CuSO4nH2O
28.274 g
Mass of Test Tube + CuSO4
26.334 g
Hypothetical Data:
Mass of Test Tube
22.960 g
Mass of Test Tube + CuSO4nH2O
28.274 g
Mass of Test Tube + CuSO4
26.334 g
Hypothetical Data:
Mass of Test Tube
22.960 g
Mass of Test Tube + CuSO4nH2O
28.274 g
Mass of Test Tube + CuSO4
26.334 g
Hypothetical Data:
Mass of Test Tube
22.960 g
Mass of Test Tube + CuSO4nH2O
28.274 g
Mass of Test Tube + CuSO4
26.334 g