SCH 3U
Experiment: Formula of a Hydrated Salt
Introduction
A hydrate is an ionic compound that contains water molecules as an integral part of its crystal
structure. The water present in the hydrate is often referred to as “water of hydration. For a given
hydrate, the number of water molecules per formula unit of the ionic compound is a whole number or a
simple fraction. Common examples include calcium sulphate dihydrate, CaSO4 · 2 H2O and copper (II)
sulphate pentahydrate, CuSO4 · 5 H2O. The water molecules are only weakly held within the hydrate
crystals, and gently heating a hydrate usually results in partial or complete removal of the water of
hydration. Complete removal of water leaves behind the ionic compound in its anhydrous form. The
term anhydrous” comes from a Greek word meaning ‘waterless.’ The compound can be returned to its
hydrated form by allowing it to cool and then adding water.
The ease of removal of water from many hydrates allows us to determine the number of water
molecules per formula unit of the ionic compound. If we heat a known mass of hydrate to drive off all
the water, then measure the mass of the anhydrous compound, the difference between these two masses
must equal the mass of water lost. Knowing the masses of the water and the anhydrous compound, we
can apply the mole concept to calculate the number of moles of each of these substances. By division,
we can find the number of moles of water per mole of anhydrous compound. The answer equals the
number of molecules of water per formula unit of the compound.
Purpose: To find the formula of a hydrated salt.
Materials
safety goggles
test tube holder
gas burner
spatula
electronic balance
hydrated metal salt
2 test tubes
CAUTION: Wear safety goggles when heating materials. Tie back loose hair and clothing. Do not
touch hot equipment. Use the proper holder.
Procedure
1. Obtain a sample of hydrated metal salt. Record the name of the salt on your labsheet and list as
many physical properties as you can.
2. Obtain a clean, dry test tube. Heat it briefly and determine its mass to the nearest 0.01 g.
3. Pour the hydrated metal salt into the test tube (~2.00g to 4.00g) and determine the mass of the test
tube and salt as carefully as possible.
4. Holding the test tube tilted at an angle heat it gently at first, and then more strongly. Continue
heating for 5 min. Place the test tube in a beaker to cool.
5. When cool, determine the precise mass of the test tube and anhydrous salt. Record the mass in your
data table.
6. Reheat and cool the test tube and contents and redetermine its mass. If the second measurement
does not agree to within 0.02 g of the first, heat, cool, and remeasure the mass of the test tube and
contents a third time. Repeat until a constant mass is obtained. Use the lowest mass of the
measurements for your calculations.
7. Examine the contents of the test tube and record your observations.
8. Place a small sample of the anhydrous salt in a small test tube and add several drops of water.
Carefully feel the bottom of the test tube. Record what you observe.
10. Empty the contents of the test tubes into the waste containers as indicated by your teacher. Clean all
equipment and return to the proper place.
SCH 3U
Experiment: Formula of a Hydrated Salt
Prelab Questions
1. Define hydrate:
2. Define anhydrous:
Quantitative Observations
1.
mass of empty test tube =
2.
mass of test tube + hydrated salt =
3.
1st mass of test tube plus anhydrous salt =
4.
2nd mass of test tube plus anhydrous salt =
5.
3rd mass of test tube plus anhydrous salt =
Qualitative Observations
Describe your observations during the experiment.
Names:
Calculations (Show all of your work for the following calculations)
1. mass of hydrated salt =
2. mass of anhydrous salt =
3. mass of water of hydration =
4. a) The name of the anhydrous salt is
b) The molar mass of the anhydrous salt is
c) How many moles of anhydrous salt were produced in the experiment?
5. How many moles of water were produced in the experiment?
6. a) How many moles of water are present in one mole of hydrated salt?
b) The formula of the hydrated salt is
c) The name of the hydrated salt is
3. a) Calculate the percent, by mass, of the anhydrate salt.
b) The accepted value for the percent, by mass of anhydrous salt is 64%. Calculate the percent error
in your experiment using the following equation.
% error = │ accepted value – experimental value │
accepted value
X 100 %
Questions: Answer the following questions.
1. How do you know that the compound left after removing the water of hydration is not the same as
the hydrated salt you started with? Give at least two reasons.
2. Reactions that produce heat are exothermic; reactions that require heat are endothermic. Indicate
one example of each reaction type from this experiment.
3. Discuss three possible sources of error in this experiment.