Name: _________________________________ Assignment #: ______ Due Date:_________
15
WATER OF HYDRATION
DETERMINING THE FORMULA OF A HYDRATE
INTRODUCTION
Many ionic compounds, when crystallized from an aqueous solution, will take up definite
amounts of water as an integral part of their crystal structure. This water of crystallization may
be driven off by heating the hydrated substance to convert it to its anhydrous form. It is then
possible to calculate the number of moles of water driven off per mole of the anhydrous
compound in a simple whole number ratio and determine the formula of the hydrate.
PURPOSE
Calculate the moles of water released by a hydrate and determine its empirical formula.
SAFETY INFORMATION
In this lab, observe all standard safety precautions, especially the ones listed below.
- Safety goggles and lab aprons must be worn at all times.
- Hot ceramic and cold ceramic look the same. Throughout the experiment, handle the
crucible with tongs.
- If your crucible breaks, DO NOT attempt to touch the pieces. Call the teacher over
for assistance as to how to proceed.
- Do not weigh a hot crucible as it may influence the mass measurement.
- Review your proper lighting procedure for a bunson burner
MATERIALS
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Bunsen burner
Sparker
Ring stand
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Iron ring
Wire gauze
Crucible
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Crucible tongs
Copper Sulfate hydrate
Glass stirring rod
PROCEDURE
1. Clamp the iron ring onto the ring stand and place the wire gauze on top. Your ring should
be only about 10 inches from countertop.
2. Place the crucible on the wire gauze. Light your Bunson burner.
3. Heat the crucible on high heat for 3 minutes. Turn off the burner and let the crucible cool for
3 minutes. (See safety information for why we do this)
4. Weigh the cooled, empty crucible; record the mass in your data table. Leave on balance
and don’t tare.
5. Weigh out about 3 g of copper(II) sulfate crystals into empty crucible on balance. Record
the combined mass of the crucible and copper(II) sulfate before heating in your data table.
6. Place the crucible with the copper (II) sulfate crystals (hydrate) onto wire gauze.
7. Relight your Bunson burner and begin heating the blue crystals. Discuss with your lab
partners and neighbors how you might tell when all of the water has been heated away.
8. While holding crucible with crucible tongs, try to break apart large chunks of the drying
crystals. Explain why you think this might be helpful in drying the copper (II) sulfate.
9. Turn off burner and allow crucible to dry for 3 minutes or so. Weigh the crucible and
anhydrous salt and record the mass in your data table.
10. Use a water bottle to squirt a few drops of water onto the anhydrous salt. Write your
observations in your data table.
Name: _________________________________ Assignment #: ______ Due Date:_________
DATA TABLE (RECORD MASSES TO TWO DECIMAL PLACES)
Mass of empty crucible (g)
Mass of crucible, and copper(II) sulfate
before heating
Mass of copper sulfate alone
before heating (g)
Mass of crucible and copper(II) sulfate
after heating (g)
Mass of water lost during heating (g)
Observations of adding water to the
anhydrous copper(II) sulfate
ANALYSIS QUESTIONS (PERFORM CALCULATIONS
TO THIS SHEET)
AND
ANSWERS
ON SEPARATE
SHEET
AND ATTACH
1. Using the experimental data obtained, determine:
a. The mass of water lost by the hydrate.
b. The number of moles of water lost.
c. The final mass of anhydrous copper(II) sulfate.
d. The number of moles of anhydrous copper(II) sulfate.
2. Using the answers obtained in 1(b) and 1(d):
a. Determine the ratio of moles of water to moles of the anhydrous copper.
b. Write the formula of the original hydrate by rounding off the above number to the
nearest whole number.
3. Consider what effect the following errors would have on the ratio of moles of water to moles
of anhydrous salt:
a. The student did not drive off all the water from the hydrate.
b. The student used a damp crucible and did not dry it before adding the hydrate.
c. After the last heating, the student allowed the crucible and contents to cool overnight
in the air before weighing.