Period: ____ Date: ______
HYDRATED CRYSTALS – Lab. Practice
Many compounds are formed in reactions that take place in water solutions. The water is then evaporated to obtain the crystalline compound. In some cases water molecules are weakly attracted to the ions or molecules that make up the compound and are retained within the crystal structure. Crystalline compounds that retain water during evaporation are referred to as being hydrated or are said to contain water of hydration. The ratio of moles of water to moles of compound is a small whole number. The formula for the hydrated compound magnesium sulfate is
MgSO4 • 7H2O
The dot shows that for every mole of MgSO4 in the crystal, there are 7 moles of H20.
The ratio of moles of H20 to moles of compound can be determined experimentally in most cases by heating to remove the water. The compound with the water removed is anhydrous. In this experiment you will determine the formula for hydrated copper(II) sulfate, CuSO4. The formula is determined by comparing the mass of the hydrated and anhydrous forms of the compound.
CuSO4 • nH2O
In this experiment, you will
• heat a specific amount of hydrated copper (II) sulfate to remove the water,
• find the experimental and theoretical percentages of water in the hydrate, and
• predict an empirical formula for hydrated copper (II) sulfate.
goggles and apron
beaker (400 or 600 cm3)
1. Prepare a data table as directed in the Analysis. Safety goggles and lab apron must be worn for this experiment.
2. Turn the hot plate to high and allow to warm up.
3. Clean a porcelain evaporating dish with soap and water. Dry the dish by placing it on the hot plate for a few minutes.
4. Carefully remove the evaporating dish with forceps and let cool. Handle the dish with forceps for the remainder of the experiment.
5. Measure the mass of the cool evaporating dish to the nearest 0.01 g.
6. CAUTION: CuSO4 is toxic and can be absorbed through the skin. Wash with water immediately if CuSO4 comes in contact with skin. Avoid breathing fumes.
Add about 3 g of CuSO4 crystals to the evaporating dish and measure the mass to the nearest 0.01 g.
7. Place the evaporating dish on a hot plate and cover with a 400 or 600 cm3 beaker as shown in Figure 8-1. Observe the inside of the beaker for a few minutes and record.
8. Carefully remove the beaker and continue heating for about 10 minutes.
9. Remove the evaporating dish from the hot plate, let it cool in a desiccator, and measure the mass.
10. Reheat the dish and contents for a few minutes, cool, and measure the mass again. If this mass value does not agree within 0.02 g with the last mass reading, you must reheat and remeasure the mass until the last two measurements are within that range.
11. After the final massing, return the CuSO4 crystals to a special container as designated by your teacher.
FIGURE 8-1. Apparatus set-up for heating hydrated crystals.
1. Prepare a table for your data, using Table 8-1 as a guide. The mass of H20 in the hydrate is the difference in mass of the hydrated and anhydrous forms of CuSO4.
A. mass of evaporating dish ______g
B. mass of evaporating dish + hydrated CuSO4 ______g
C. mass of hydrated CuSO4______g
D. mass of evaporating dish + anhydrous CuSO4 ______g
E. mass of anhydrous CuSO4 ______g
F. mass of H20 in the hydrate______g ratio:
G. moles of anhydrous CuSO4 ______molI. __:__
H. moles of H20 in the hydrate ______mol
(a) Determine the percentage of water in the hydrated crystals of CuSO4, using your data.
(b) Assume that the correct formula is CuSO4 • 5H2O. Using this formula, calculate the theoretical percentage of water in the hydrated crystals.
Theoretical percentage of H2O =
(c) Compare the experimental percentage of water in the hydrate with the theoretical percentage and calculate your percentage of error.
Percentage error =
1. Compare the number of moles of anhydrous CuSO4 to the number of moles of water in the hydrate. Use the ratio of these two values to predict a formula for hydrated CuSO4.
2. The method used in this experiment to find the percentage of water in the hydrated crystals is not suitable for all hydrates. Give at least two reasons why this may be so.
(a) Why is it necessary to let the evaporating dish cool before measuring mass?
(b) Why should the mass of the evaporating dish be measured immediately after it cools, and not later?
4. Compare your results with the results of other members of your class. Are there differences? If so, give reasons for the differences.
5. How would your experimental results be affected if you did not use a desiccator when cooling the crucible and contents?
FURTHER INVESTIGATIONS (Extra) (Include references)
1. Write a brief report on some of the other methods used to determine the percentage of water in a hydrate.
2. In this experiment you cooled your evaporating dish in a desiccator. What is a desiccator? How does a desiccator work?
3. Why is knowing the water content of substances important?