Oxford Area High School Science Department

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Oxford Area High School
Science Department
• Experiment 30 Water in a Hydrate
Water in a Hydrate Lab 30
Text reference: Chapter 10
Introduction:
Many compounds are formed as a result of reactions that occur in water solutions. These compounds appear to be
dry, but when they are heated, large amounts of water are released The water molecules ate part of the crystalline structure
and are weakly bonded to the Ions or molecules that make up the compound Such compounds are known as hydrates,
meaning that they contain water, The solid that remains when the water is removed Is referred to as the anhydrous salt, or
anhydrate.
Hydrate Anhydrate + H2O
Usually, the amount of water present in a hydrate is in a whole number molar ratio to the moles of anhydrate. An
example of a hydrate is magnesium sulfate. Its formula is MgSO4•7H2O, indicating that seven moles of water are combined
with one mole of magnesium sulfate in the crystalline form.
In this investigation you will be given an unknown hydrate and asked to determine the percent of water in the
compound. From this information, the molar ratio of water to anhydrous salt will be calculated. Finally, the identity of the
hydrate will be determined.
Pre-Lab Discussion: Read the entire laboratory investigation and the relevant pages of your
textbook. Then answer the questions that following
1. What is a hydrate? An anhydrate?
2. Why do you think it is necessary to heat the evaporating dish before
finding its mass?
3. Why must the evaporating dish be cooled before finding its mass?
4. Why must the mass of the anhydrous salt be measured immediately
on cooling?
5. Why is it necessary to handle the evaporating dish only with the crucible tongs after the initial heating?
6. What is the molar mass of the hydrate of magnesium sulfate described in the Introduction?
7. What is the molar mass of the anhydrous salt, magnesium sulfate?
8. What is the percent of water In MgSO4•7H2O?
Problem: How can the percent of water in an unknown hydrate be determined?
Materials:
chemical splash goggles
laboratory apron
pipe-stern triangle ring stand
Iron ring
lab burner
matches
crucible with cover
crucible tongs
heat-resistant pad
laboratory balance
spatula
one of the following hydrated salts
magnesium sulfate heptahydrate (MgSO4•7H2O)
sodium carbonate monohydrate (NaCO3•1 H2O)
zinc sulfate dihydrate (ZnSO4•2 H2O)
manganese sulfate mono-ydrate (MnSO1•1 H2O)
calcium sulfate dihydrate (CaSO4•2H2O)
copper sulfate pentahydrate (CuSO4•5 H2O)
Safety:
Wear your goggles and lab apron at all times during the investigation The crucible and its cover are hot. Move them only with
the tongs. Tie beck loose hair and clothing when working with a flame You should assume that all of the hydrates are poisonous.
If they contact your skin or clothing, wash the area with plenty of water. Note the caution alert symbols here and with certain
steps of the Procedure. Refer to page xi for the specific precautions associated with each symbol.
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or clothing
when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it forms.
See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the heat
momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Figure 30-1
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01
g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher.
Clean up your work area and wash your hands before leaving the Laboratory.
Figure 30-1
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes.
CAUTION: Tie back loose hair or clothing when working with the flame. The crucible and cover are very hot.
Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest
0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the
cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the heat
momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover,
and hydrate to the nearest 0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow
water vapor to escape as it forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there
is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Figure 30-2
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01 g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to
elevate. Allow it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover,
and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Notebook
Procedure:
1. Put on your goggles and lab apron.
2. Prepare the setup shown in Figure 30-1
3. Light the burner and heat a dean, dry crucible with its cover in the hottest part of the flame for 3 minutes. CAUTION: Tie back loose hair or
clothing when working with the flame. The crucible and cover are very hot. Use tongs to move them.
4. Remove the crucible and cover and place them on a heat- resistant pad to cool lot at least 3 minutes.
5. Measure and record the mass of the cool crucible and cover to the nearest 0.01 g.
6. Add about 2 g of your unknown hydrate to the crucible. Measure and record the mass of the crucible, cover, and hydrate to the nearest 0.01
g.
7. Place the crucible, cover, and hydrate on the pipe-stem triangle. The cover should tilt slightly, which will allow water vapor to escape as it
forms. See Figure 30-2. Begin heating gently, gradually increasing the heat until there is no more popping or spattering.
8. Remove the cover using the tongs and examine the material in the crucible. If the edges of the solid appear to be turning brown. Reduce the
heat momentarily and then begin heating again at a slower rate. Heat for 5 more minutes.
9. Remove the crucible and place it on the heat-resistant pad. CAUTION: The crucible is very hot. Use tongs to elevate. Allow
it to cool for at least 1 minute. Immediately measure and record the mass of the crucible, cover, and anhydrous salt.
10. Reheat the crucible, cover, and contents for a few minutes cool and measure the mass again the value should be within 0.02 g of the last
recorded mass. If it is not, reheat and remeasure the mass until the last two measurements axe within that range. Record the final mass.
11. Dispose of the reaction product as directed by your teacher. Clean up your work area and wash your hands before leaving the Laboratory.
Notebook
Observations:
DATA TABLE
mass of crucible and cover
mass of crucible, cover, and hydrate
mass of crucible, cover, and anhydrate (after Step 9)
mass of crucible, cover, and anhydrate (after Step 10)
Calculations:
1. Calculate the initial mass of hydrate.
2. Calculate the mass of water lost.
3. Calculate the number of moles of water lost.
4. Calculate the mass of the anhydrous salt.
Critical Thinking: Analysis and Conclusion:
1. Based on the molar mass of anhydrous salt given by the teacher, calculate the moles of anhydrous salt determine the smallest anhydrous
salt. (Interpreting data)
2. After consulting the list of salts in the Materials section, determine the empirical formula for your compound whole number ratio of moles
of water to moles of anhydrate. (Drawing conclusions)
3. What is the percent water in your hydrate? What is the percent error for this investigation? (Interpreting data)
4. The method used in this investigation is not suitable for all hydrates, Give two reasons for this. (Making inferences)
5. Compare your results with other members of your class with the same unknown. Are there differences? What reasons could account for
these differences? (Making comparisons)
Critical Thinking Applications:
1. Explain what effect the following errors would have on the value for the percent water in the hydrate (Making inferences)
a. The hydrate was not heated long enough to drive off all the water.
b. A damp crucible was used, and it was not dried before adding the hydrate.
c. The crucible and contents were allowed to cool overnight before finding their mass.
2. Examine some of the different hydrate crystals with a magnifying glass. Describe the shapes of the crystals. How do the crystals of one
compound differ from the crystals of another? (Making comparisons)
3. Predict what would happen if you added a few drops of distilled water to the anhydrous salt remaining at the end of the experiment. (Making
Predictions)
Notebook
Figure 30-1
Return to procedure
Figure 30-2
Return to procedure
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