Name
Date
Water in a Hydrate
_ Class
_
Lab13
Text reference: Chapter 8
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 are 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 + Heat

Anhydrate + H20
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 Nickel(II) sulfate. Its formula is NiS04 •6H20, indicating that six
moles of water are combined with one mole of nickel(II) sulfate in the
crystalline form.
In this investigation you will be given one known and one unknown
hydrate and asked to determine the percent of water in each compound. From
this information, the molar ratio of water to anhydrous salt will be calculated.
Finally, the identity of the unknown hydrate will be determined.
Pre-Lab Discussion
Read the entire laboratory investigation and the relevant pages of your
textbook. Then answer the questions that follow in your lab notebook.
1. What is a hydrate? An anhydrate?
2. Why must the aluminum foil dish be cooled before finding its
mass?
4. Why must the mass of the anhydrous salt be measured immediately
on cooling?
.
Water in a Hydrate 157
Name
5. Why is it necessary to handle the aluminum foil only with the
tongs after the initial heating?
6. What is the molar mass of the hydrate of nickel (II) sulfate described
in the Introduction?
7. What is the molar mass of the anhydrous salt, nickel (II) sulfate?
8. What is the percent of water in NiSO4 • 6H2O?
Problem
How can the percent of water in an unknown hydrate be determined?
Materials
chemical splash goggles
laboratory apron
pipe-stem triangle
ring stand
iron ring
lab burner
matches
evaporating dish
aluminum foil
tongs
laboratory balance
spatula
one of the following hydrated
salts:
magnesium sulfate heptahydrate (MgSO4 •7H20)
sodium carbonate monohydrate (Na2C03 •lH20)
zinc sulfate dihydrate
(ZnSO4 •2H2O)
manganese sulfate monohydrate (MnSO4•1H2O)
calcium sulfate dihydrate
(CaS04 •2H20)
copper sulfate pentahydrate
(CuS04 •5H20)
Safety
Wear your goggles and lab apron at all times during the investigation.
The evaporating dish and aluminum foil are hot. Move them only with the
tongs. Tie back 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.
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Name
_
Lab
13
Procedure
1 . Put on your goggles and lab a p r o n .
2. P r e p a r e t h e s e t u p s h o w n i n F i g u r e 3 0 - 1 . .
Figure 30-1
•
3
3. Make one dish and lid with aluminum foil as demonstrated by
your teacher that will fit neatly in the evaporating dish and record
the mass of the each aluminum foil dish with cover to the nearest 0.01 g.
Record your data in data table 1.
4. Add about 2 g of your known hydrate to the aluminum foil dish.
Measure and record the mass of the aluminum foil dish, cover, and hydrate
to the nearest 0.01 g.
5. Place the dish, cover, and hydrate in the evaporating dish. 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.
Figure 30-2
'Y
j
6. Remove the cover using the tongs and examine the material in
the dish. 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.
7. Remove the aluminum dish and place it on the lab bench.
CAUTION: The dish is very hot. Use tongs to move it. Allow
it to cool for at least 1 minute. Immediately measure and record
the mass of the aluminum dish, cover, and anhydrous salt.
8. Reheat the dish, cover, and contents for a few minutes, cool
and measure the mass again. The value should be within 0.02 grams
of the last recorded mass. If it is not, reheat and re-measure the
mass until the last two measurements are within that range.
Record the final mass.
9. Repeat steps 3-9 with your unknown sample. Be sure to record
your results in data table 2.
10. Dispose of the reaction product as directed by your teacher.
Clean up your work area and wash your hands before leaving
the laboratory.
Observations
DATA TABLE 1 (Known hydrate)
Mass of aluminum foil dish and lid
Mass of aluminum foil dish, lid, and hydrate
Mass of aluminum foil dish, lid, and anhydrate (after 1st
heating, step 7)
Mass of aluminum foil dish, lid, and anhydrate (after
2nd heating, step 8)
DATA TABLE 2 (Unknown hydrate)
Mass of aluminum foil dish and lid
Mass of aluminum foil dish, lid, and hydrate
Mass of aluminum foil dish, lid, and anhydrate (after 1st
heating, step 7)
Mass of aluminum foil dish, lid, and anhydrate (after
2nd heating, step 8)
Calculations (for both the known and unknown)
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.
5. Calculate the moles of anhydrous salt. (For unknown calculation, you
will need to get the molar mass of your unknown from the teacher)
6. Determine the molar ratio of water to anhydrate.
Critical Thinking
· 1 Based on the list of hydrates in the Materials section, do the
calculations for you known compound make sense? Explain and
show calculations.
2. After consulting the list of salts in the Materials section, what is the
formula for your unknown compound?
3. What is the percent composition of water in your hydrate? What is
the percent composition of water in your unknown?
4. Based on your percent of water from question 3, Calculate the percent error
for both your known and unknown hydrate.
| measured value – true value | x 100 = Percent Error
true value