Hydrates have a variety of practical applications. Their ability to gain or lose their waters of hydration makes them versatile. One formula unit of a hydrate contains one formula unit of an anhydride bonded to a fixed number of water molecules. Careful heating removes the water so that the ratio of water molecules to anhydride formula units can be determined, provided the molar mass of the anhydride is known. A hydrate is represented by the formula of the anhydride followed by a raised dot which represents the "weak" bond between the anhydride and the number of water molecules, i.e. MgSO₄•7H₂O.

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To determine the formula of a blue hydrate.

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Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Dispose of all solutions in the containers provided by your teacher. Wash your hands before leaving the laboratory.

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Perform this activity with a partner. Prepare a table for recording data. Record all mass measurements to the nearest 0.01 g.

1. Determine the mass of an 18 x 150 mm unscratched, hard-glass test tube.
2. Add 2 to 4 g of blue hydrate to the test tube. Find the mass of the test tube plus hydrate.
3. Tilt the test tube, spreading the hydrate so it covers about half the length of the test tube.
4. Use a test tube holder to hold the test tube nearly horizontal; tilt it with the mouth slightly downward. Move the test tube back and forth through a relatively cool (no distinct inner cone) burner flame. (Caution: Avoid pointing the mouth of the tube at anyone.)
5. Continue heating until all liquid disappears and only a finely divided, light-colored powder remains. Chase the water toward the mouth of the test tube. Avoid overheating. (You are overheating if the solid begins to turn light brown.) The light-colored, powdery residue is the anhydride. Record other observations during the heating.
6. After allowing the anhydride to cool, determine the combined mass of the anhydride and test tube.
7. Reheat the anhydride for approximately one minute. Cool. Weigh. If this mass differs by more than 0.05 g from the mass obtained in Step 6, repeat the heating, cooling, and weighing process.
8. Add about 1 mL of water to the anhydride. Observe. Feel the outer wall of the test tube.
9. Empty the test tube contents into the container designated by your teacher.
10. Wash hands thoroughly before leaving the laboratory.

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1. Calculate the initial mass of the blue hydrate sample.
2. Using the final mass of test tube and product, calculate
   1. the mass of water lost and
   2. the mass of white anhydride remaining.
3. Calculate the moles of water lost.
4. Calculate the moles of anhydride, using the molar mass of anhydride supplied by your teacher.
5. Calculate the number of water units for each anhydride unit.
6. Write the formula of the blue hydrate in the form A.xH₂O, where A represents the formula of the anhydride and x is the number of water units expressed as an integer.
7. 1. Calculate the mass percent of water in your original hydrate sample using the mass of water lost and the mass of the blue hydrate.
   2. The accepted value is 36.1%. What is your percent error? Were you successful in converting the hydrate completely to the anhydride?
8. Draw a particulate model of the blue hydrate and the anhydride using a diamond to represent the anhydride and a circle to represent H₂O.
9. Use the pictures in Question 8 to explain your observations in Step 8.

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1. If the color of the hydrate were identical to the color of the anhydride, how could you be sure that you had driven off all the water from the hydrate?
2. How could you use the anhydride of this blue hydrate to indicate whether the humidity in the atmosphere were high or low?
3. Solar heating systems utilize various substances for heat storage. Design a solar-powered system that uses the heat-releasing and heat-absorbing properties of this anhydride/hydrate system to keep a room warm after sunset.

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Teachers Guide

Preparing for the Laboratory Activity

• Major Chemical Concept
• Level
• Expected Student Background:
• Time
• Safety
• Materials
• Advance Preparation

Conducting the Laboratory Activity

• Pre-Lab Discussion
• Teacher/Student Interaction
• Anticipated Student Results
• Answers to Data Analysis
• Answers to Imply, Apply
• Post-Lab Discussion
• Possible Extensions

Assessing the Laboratory Learning

• Paper/Pencil Items
• Answers to Paper/Pencil Items
• Check List
• Laboratory Report

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Preparing for the Laboratory Activity

Major Chemical Concept

Empirical formulas may be determined experimentally.

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General and advanced chemistry

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The student should be able to:

• Use a Bunsen burner
• Calculate moles, given mass of a substance
• Calculate molar mass, given the formula for a substance
• Calculate percent error
• Determine the mass of a sample to proper precision

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50 min

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• Copper(II) sulfate is poisonous. Avoid breathing dust or handling crystals.
• Remind students that the mouth of the test tube being heated should not point toward anyone.
• The mouth of the test tube should be tilted slightly downward to prevent the water from running back and breaking the hot test tube.
• At the end of the activity, have students empty their solutions into designated containers.

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Non-Consumables (per lab team)

• 1 Bunsen burner
• 1 Spatula
• 1 Test tube holder
• 1 Test tube, 18 X 50 mm, hard glass, unscratched

Consumable (per lab team)

• Copper(II) sulfate pentahydrate, CuSO₄•5H₂O, 4 g

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1. Label six wide-mouth containers "blue hydrate" and fill halfway with copper(II) sulfate pentahydrate, CuSO₄•5H₂O. Keep containers covered. Small crystals are less likely to result in decomposition of the anhydride (formation of CuO) but large crystals are more exciting visually. Keep bottles closed.
2. Prepare several 150 mL labeled beakers for collection of the final solution.

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Conducting the Laboratory Activity

Pre-Lab Discussion

This discussion must occur on the day preceding the activity since the procedure requires an entire class period itself. Demonstrate a "gentle flame" -- about 2 cm high with no definite inner cone. Review, as needed, data table preparation, determining mass by difference, and heating to constant mass. Remind students of the uncertainty in measurement when comparing the mass data produced by the two heatings (Step 8 of Procedure). Demonstrate holding the test tube nearly horizontal, mouth downward, while moving it back and forth in the flame.

If this is your students' first experience with particulate models, explain how to represent the particles using drawings.

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While walking among the students, probe their understanding of the Law of Conservation of Matter. (Why is the mass less after heating the hydrate? Where did the water go? Is the water recoverable?) Check that the differences between the two post-heating masses are within the range specified in the procedure and that students do not confuse the difference between these two values with the difference between the mass before and after heating. Ask students to describe to you the "pictures in their minds" of the particles involved in this activity. Check that they understand possible effects on results if incorrect techniques are used (if the test tube were not cooled before weighing, if tap water were added instead of distilled water in Step 9, etc.).

Note that students will need the molar mass mass of the anhydride to complete Data Analysis, Concept Question 4. The value is 160 g/mol.

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Sample Data Mass (g)

1. Test tube 18.83
2. Test tube & hydrate 21.70
3. Test tube & hydrate:

1st weighing: 20.68
2nd weighing: 20.68
3rd weighing not needed

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1. Initial mass of hydrate (21.70 g - 18.83 g = 2.87 g)
2. 1. Mass of H₂O lost (21.70 g - 20.68 g = 1.02 g)
   2. Mass of hydrate (20.68 g - 18.83 g = 1.85 g)
3. Moles of H₂O lost (1.02 g (1 mol/18.0 g) = 0.0567 mol)
4. Moles of anhydride (1.85 g (1 mol/159.5) = 0.0116 mol)
5. 0.567 mol H₂O/0.0116 mol anhydride = 4.89/1.00
6. A.5H₂O
7. 1. Mass percent of H₂O ((1.02 g/2.87 g) x 100 = 35.5%)

   Percent error [(36.1 - 35.5)/36.1] x 100 = 1.66%

8. See pentahydrate diagrams above. Circles must be attached in blue hydrate, circles separated from diamonds for anhydride and water.
9. When water molecules separate from anhydride, the hydrate color changes. When water is added to the anhydride, the water molecules reattach, reforming the hydrate and restoring the blue color. Similarly, adding water to the anhydride releases the heat absorbed during the initial separation process. Breaking the attachment of the water molecules to the anhydride absorbs heat. Reattachment of water molecules to the anhydride releases heat.

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1. After reheating and obtaining constant mass, the student could be confident that the product is composed only of the anhydride.
2. When the white anhydride is placed in a humid environment, formation of the pale hydrate can be easily observed. The intensity of blue coloration could then be used as a measure of humidity.
3. By utilizing a closed system, solar energy could be used to form the anhydride by vaporizing the water molecules contained in the blue hydrate. At a lower temperature, the blue hydrate could reform. The exothermic reaction could then provide energy to warm a room after sunset.

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Discuss the results including the constancy of percent water, 36.1% for CuSO₄•5H₂O. You may wish to use student values to calculate the class average or draw a histogram of their results. If you use a computer spreadsheet to analyze data, student-entered data will readily yield a mean value for the ratio. An overlay made from a spreadsheet printout would help students see that their values all cluster about a mean of 5 for the water-to-anhydride ratio.

Solicit comments and explanations for color changes observed, sounds heard, heat evolution felt, and moisture observed. (Students may even observe a flame test for copper at the mouth of the test tube.) Be sure to stress the particulate nature of matter in all of these explanations.

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1. To reinforce the reversibility of this process, pour final solutions into petri dishes and allow the water to evaporate. Students will enjoy watching the beautiful crystals form over time.
2. There are two hydrates of calcium sulfate whose formulas contain different water-to-anhydride ratios. Students may investigate how these are related to the solidification of Plaster of Paris used to cast broken bones.
3. By taking advantage of the low solubility of copper(II) sulfate anhydride and copper(II) sulfate pentahydrate in alcohol, alcohol can be dehydrated by adding the anhydride. Removal of the hydrate can then be accomplished by filtration.
4. Investigate the use of anhydrides as drying agents in cookie and potato chip dehumidifiers.
5. If calorimetry has been studied previously, have students devise a method of measuring the heat of reaction for the hydration of copper(II) sulfate anhydride. Remind them that the dissolving process itself could release or absorb energy and must be taken into account in the experimental design.

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Assessing the Laboratory Learning

Paper/Pencil Items

Allow students to use their graded lab reports while completing the following assessment.

1. Draw particulate model sketches for conversion of pentahydrate to its anhydrous salt, using a diamond for anhydride and a circle to represent water.
2. Which process released heat energy?
3. Which process absorbed heat energy?
4. If the hydrate is overheated, it turns blackish brown. The anhydride itself decomposes into a gas and a solid. If a student overheats the hydrate, how will the ratio of moles of water to moles anhydride change?
5. If the student underheats the hydrate, what will happen to the observed ratio of moles water to moles anhydride?
6. Write a paragraph describing your sensory perceptions of this chemical reaction.
7. In a similar activity, a 3.60 g hydrate sample released 0.54 g water. The molar mass of the anhydride is 204 g/mol. Calculate the empirical formula of the hydrate.
8. A student obtained the following laboratory data:

   Mass of test tube and blue hydrate = 20.21 g

   Mass of empty test tube = 17.20 g

   Mass of test tube and light-colored product = 19.47 g

   At this point, the student was called to the Guidance Office. When she returned to the lab at lunch, she reheated the test tube and light colored product. After reheating and cooling, the product was still light colored and had a mass of 19.13 g. She reheated and cooled the product again and determined that the mass was unchanged (19.13 g).

   1. Was the student wise to return at lunchtime? Explain your reasoning using only information from the student's data.
   2. Using both information from this student's data and your own laboratory data, draw a particulate model of the blue hydrate at the start (as above) and when she left for guidance. (Recall: a diamond for the anhydride and a circle for water.)

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1. See earlier diagrams
2. Blue hydrate + heat --> anhydride + water
3. Anhydride + water --> blue hydrate + heat
4. Increases
5. Decreases
6. Brilliant blue, shiny crystals, when heated, turn to an almost white (dull green) powder. Water droplets gather at the neck of the test tube as the reaction progresses. The escaping water vapor turns blue-green when the flame touches it. When water is added to the cool, powdery anhydride, so much heat is released that the water fizzes audibly. In addition, the test tube becomes hot to touch, and the powder turns bright blue again as the hydrate reforms.
7. Mass of anhydride = 3.60 g - 0.54 g = 3.06 g. Moles of anhydride = 3.06 g x 1 mol/204 g = 0.0150 mol. Moles of water = 0.54 g x 1 mol/18.0 g = 0.030 mol. Ratio = 0.030 mol/0.015 mol = 2.0. Empirical formula = A•2H₂O.
8. 1. She was wise to return, since all the water had not been vaporized earlier from the hydrate.
   2. See above diagrams.

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List the following techniques on the chalkboard. Point out to students that they will be graded on each item during the laboratory period. All items are of equal importance.

• Bunsen burner flame adjustment
• Proper placement of tube in flame
• Gradual tube movement
• Proper color change of hydrate
• Lack of discoloration of anhydride
• Proper disposal and clean-up

To facilitate checking these techniques, list the skills along the top of a grid. List lab partners/lab stations down the left-side of grid so that visual evaluations are easily recorded.

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In evaluating the student reports, use a check list to assess "whole number" ratio, conservation of matter, and achievement of constant mass.

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