LAB # 3.5
Lab: Percent Water in a Hydrate
Objectives
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Analyze the percent water in a crystalline hydrate
Identify the hydrate from a list of possible unknowns
Introduction
A compound is a pure substance - it has a fixed (constant) composition. The composition of a pure substance
is the same throughout and does not vary from one sample to another. According to the law of definite
proportions, a compound always contains the same elements in the same proportions, regardless of the
amount of the sample, where it was found, or how it was prepared. A mixture, on the other hand, may contain
variable amounts of different substances. The composition of a mixture is not constant.
A hydrate is a pure substance that contains water molecules embedded in its crystal structure. Heating a
hydrate "drives off" the water molecules, and the solid that remains behind is called anhydrous, meaning
"without water." The chemical formula of a hydrate specifies the relative number of each kind of atom in a
formula unit of the compound, as well as the number of water molecules bound to each formula unit. Calcium
chloride dihydrate, which is used as road salt, is an example of a hydrate. The chemical formula for calcium
chloride dihydrate is CaCl2•2H2O. The "dot" in the chemical formula indicates that two water molecules (H2O)
are attached or bound to the ions in solid calcium chloride (CaCl2) by which chemical bonds. The water in
calcium chloride dihydrate can be removed by heating the hydrate (Equation 1).
heat
CaCl2•2H2O
147.02 g
dihydrate
→
CaCl2
110.98 g
anhydrous salt
+ 2H2O
Equation 1
36.04 g
The number of water molecules in a hydrate is called the water of hydration. The water of hydration of
calcium chloride dihydrate is two water molecules per every one formula unit of calcium chloride. The number
of water molecules in a typical hydrate is characteristic of that particular salt and is usually a small whole
number from 1 to 10. The chemical formula of a hydrate can be determined by analyzing the percent water
in the hydrate - the ratio of the mass of the water of hydration lost upon heating divided by the mass of the
original hydrate (Equation 2).
mass of water of hydration
x 100 = percent water in hydrate
Equation 2
mass of hydrate
1
Introduction Continued:
The formulas of some common hydrates and their anhydrous salts are summarized in Table 1.
Table 1. Formulas of Hydrates and their Anhydrous Salts
Common
Chemical
Hydrate
name
name
Washing soda
Gypsum
Epsom Salt
Sodium carbonate monohydrate
Calcium sulfate dihydrate
Magnesium sulfate heptahydrate
Na2CO3•H2O
CaSO4•2H2O
MgSO4•7H2O
Anhydrous
Salt
Na2CO3
CaSO4
MgSO4
Materials
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Chemical-resistant goggles, apron, and gloves
Chemical scoop
Distilled water
Electronic gram balance
Laboratory burner and tubing
Ring stand
Support stand
Sparker
Test tube (Pyrex®), medium
Test tube clamp, uncoated
"Unknown" crystalline hydrate
Weigh boat
Wire gauze or heat-resistant ceramic pad
Safety Precautions
Recall the general safety rules for working with a laboratory burner. Keep long hair tied back, make sure that
hair, clothing and hands are a safe distance from the flames at all times, and always wear goggles and closedtoe shoes in case of spattering. Never reach over the top of a burner flame. Light the burner only as instructed
by the teacher and do not leave a lit burner unattended. Always turn the burner off when not in use. Use only
heat-resistant, borosilicate glassware(e.g., Pyrex®). Heated materials remain hot for a very long time, and
HOT GLASS LOOKS EXACTLY THE SAME AS COLD GLASS. Place the test tube on a wire gauze or on a
heat-resistant pad to cool. The crystalline hydrates used in this lab are slightly toxic by ingestion and may be
irritating to skin and eyes. Avoid contact of all chemicals with eyes and skin. Wear chemical-splash goggles,
chemical-resistant gloves, and a chemical-resistant apron. Wash hands thoroughly with soap and water before
leaving the lab.
Answer the pre-lab questions found at the end of this lab
packet. Tear off the answer sheet and turn it in by:
_________
2
Procedure
1.
Observe the *color and appearance of the unknown hydrate and record the observations,
along with the label code, in the data table on the Post Lab section.
2. Set up a support stand with an uncoated test tube clamp.
3. Adjust the height of the test tube clamp so that a laboratory
burner can be moved freely under the test tube during heating
(step 8). See Figure 1.
4. Measure the mass of a clean, dry test tube to the nearest
0.001 g. Record the mass in the data table in the
Post Lab section.
5. Use a chemical scoop to transfer about 2 grams
of unknown crystalline hydrate to the test tube.
6. Mass the combined mass of the test tube and the
unknown crystalline hydrate. Record this mass in
the data table in the Post Lab section.
7. Hold the test tube almost horizontally and
GENTLY tap the test tube until the hydrate
is spread out over the bottom one-third portion
of the test tube (Figure 1). Note: Some hydrates
will spatter when they are heated.
8. Insert the test tube into the test tube clamp
being careful to keep the test tube almost
horizontal with the open end slightly elevated.
9. Attach the test tube clamp to the ring stand
again being careful to keep the test tube
almost horizontal with the open end slightly
elevated (Figure 1).
10. Light the laboratory burner and "sweep" the bottom of the test tube with the top of the burner flame.
Continue to sweep the flame along the portion of the test tube containing the hydrate for about 4-5
minutes.
11. Periodically brush the flame over the top and mouth of the test tube as well to evaporate any water
droplets that collect there.
12. Turn off the burner flame when there is no more water vapor escaping from the opening of the test tube
and no further changes are observed in the appearance of the test tube contents. Note: For most
hydrates, 5 minutes of heating (steps 10 and 11) will be sufficient. However, for a few hydrates, up to
15 minutes of heating will be required.
* examples: white large crystals, or blue powder, or yellow small crystals, or orange granular
3
Procedure Continued:
13. Allow the test tube to cool completely for 8-10 minutes. Note: The test tube is cool enough to weigh if
you do not feel any radiant heat when placing your hands above the test tube.
14. Measure the combined mass of the test tube and the anhydrous residue. Record this mass in the data
table in the Post Lab section.
15. Observe the color and appearance of the anhydrous residue and record the observations in the data table
in the Post Lab section.
Clean Up
Dispose of the contents of the test tube into one of waste disposal containers in the fume
hood. DO NOT throw the residue in the trash can!!!
4
Name __________________________________ Date ____________________ Period ______
POST-LAB QUESTIONS for LAB 3.5:
Percent Water in a Hydrate
Data Table:
"Unknown" hydrate label Code(Step 1)
Color and Appearance of unknown Hydrate (Step
1)
Mass of test tube (Step 4)
Mass of test tube and hydrate (Step 6)
Mass of test tube and anhydrous residue (Step 14)
Color and Appearance of anhydrous residue (Step
15)
1.
Calculate the following. Follow ALL math work rules!
(a) the original mass of the hydrate:
(b) The mass of the water of hydration:
(c) the percent water in the hydrate:
5
Lab #3.5 Post-Lab Question Continued:
2. The unknown hydrate is one of the following substances. Complete the table to calculate the theoretical
percent water for each hydrate. You do NOT need to show any work.
3.
CuSO5•5H2O
MgSO4•7H2O
BaCl2 •2H2O
Sum of atomic masses
(anhydrous salt)
159.61
120.38
208.23
Sum of atomic masses
(nH2O)
90.10
Sum of atomic masses
(hydrate)
Percent of water in hydrate
(theoretical)
4.
(a) What is the probable identity of the unknown crystalline hydrate?
(b) Explain your reasoning.
4. Assume you have correctly identified the unknown hydrate. Calculate your percent error in
the percent water analysis. Use the following equation and follow ALL math work rules!
theoretical value - experimental value
percent error =
x 100
theoretical value
6
Lab #3.5 Post-Lab Question Continued:
5.
(a) Is the conversion of the hydrate to its anhydrous salt a physical or a chemical change?
(b) Explain.
6.
(a) Is the unknown hydrate a mixture or a pure substance?
(b) Explain.
6. Consider the following potential sources of error in this experiment.
______ (a) water was observed around the mouth of the test tube after heating
______ (b) the anhydrous residue decomposed upon heating, liberating gas
______ (c) the hydrate spattered during heating and some of the sample was lost.
7
For each of the potential sources of error above indicate whether the percent water calculated in question 1(c)
would be too high (H), too low (L), or unchanged (NC).
Name __________________________________ Date ____________________ Period ______
PRE-LAB QUESTIONS for LAB 3.5
Percent Water in a Hydrate
1.
State the reason that the glassware being used during a heating lab should always be assumed to be hot?
2.
Compare the composition of a compound with that of a mixture.
3.
a) According to the introduction to this lab, what is the law of definite proportions.
b) Give a chemical example and explain WHY it obeys the law of definite proportions.
4. Define each of the following:
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a) hydrate
Lab # 3.5 Pre-Lab Questions Continued:
4. Contiued: Define each of the following:
b) anhydrous salt
c) water of hydration
5. Calculate the percent water in calcium chloride dihydrate. Follow ALL math work rules!
6. Briefly list three (3) general safety rules for working with a laboratory burner.
9
Lab # 3.5 Pre-Lab Questions Continued:
7. The following data were obtained when a sample of barium chloride hydrate was analyzed as described
in the Procedure section.
Mass of empty test tube
Mass of test tube and hydrate (before heating)
Mass of test tube and anhydrous salt (after heating)
18.42 g
20.75 g
20.41 g
a) Calculate the mass of the hydrate. Follow ALL math work rules!
b) Calculate the mass of the water of hydration. Follow ALL math work rules!
c) Calculate the percent water in the hydrate. Follow ALL math work rules!
8. The general formula of barium chloride hydrate is BaCl2•nH2O, where n is the number of water
molecules. Calculate the theoretical percent water for each value of n - divide the sum of the atomic
masses due to the water molecules by the sum of all the atomic masses in the hydrate, and multiply the
result by 100. Complete the table. You do NOT need to show any work - but make sure you do the
work yourself so that you know HOW to do the work!
BaCl2
BaCl2•H2O
BaCl2•2H2O
BaCl2•3H2O
208.23
208.23
208.23
208.23
0
18.02
Sum of atomic masses (BaCl2)
Sum of atomic masses (nH2O)
Sum of atomic masses (hydrate)
10
Percent water in
hydrate(theoretical)
208.23
226.25
0%
7.96%
Lab # 3.5 Pre-Lab Questions Continued:
9. Compare the percent water in the hydrate, question 7(c), with the theoretical values calculated for the
different values of n in question 8.
a) What is the most likely formula for barium chloride hydrate? ___________________________
b) Explain your choice.
10.
TRUE / FALSE
(circle one)
11. TRUE / FALSE
(circle one)
Since water of hydration is not liquid, there is no need to worry
about spattering as the unknown crystalline hydrate is heated.
Almost ALL hydrate must be heated for up to 15 minutes to
completely evaporate all the water of hydration.
12. Explain how you will know that the heated test tube is cool enough to mass the anhydrous
residue.
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