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Laboratory 8
Determination of the
Formula of a Hydrated Crystal
Introduction:
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 copper(II) sulfate is
CuSO4  5H2O
The dot shows that for every mole of CuSO4 in the crystal, there are 5 moles of H2O.
The ratio of moles of H2O 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 barium chloride. The formula is determined
by comparing the mass of the hydrated and anhydrous forms of the compound.
Equipment:
Crucible and cover
Clay triangle
Crucible tongs
Ring stand and iron ring
Laboratory burner
Desiccator
Procedure:
1. Prepare a data table for all physically measured values.
2. Clean a crucible and cover with soap and water. Dry the crucible and cover by placing the
crucible and cover on a clay triangle over a laboratory burner and heating for two to three
minutes. See figure 1.
Crucible Cover
Crucible Tongs
3. Carefully remove the crucible and cover with crucible
tongs and let cool on wire gauze. Handle the crucible and
cover with tongs for the remainder of the experiment.
4. Mass to the nearest 0.01-g the empty crucible and cover.
Record the mass.
5. Obtain about 3-g of BaCl2 crystals and place in it the
crucible, replace the cover. Measure the mass of the
crucible, the cover and the hydrate crystals and record.
Figure 1
CAUTION: Barium chloride is poisonous and can be
absorbed through the skin. Wash with water immediately if BaCl2 comes in contact with skin.
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6. Place the crucible and cover in the clay triangle, move the cover so it doesn't completely cover the
crucible. Begin heating slowly. Increase the heat until you have heated the crucible strongly for
10 to 15 minutes.
7. Remove the crucible from the ring stand, place the cover back on the crucible, and place the set
up in a desiccator to cool. Measure the mass of the crucible, cover and anhydrous salt.
8. Reheat the crucible, cover and salt for a few minutes. Allow it to cool in the desiccator. Measure
the mass.
9. If the mass of the crucible, cover, and anhydrous salt in step 8 is less than the mass in step 7 by
more than 0.02-g repeat step 8 until the masses agree within 0.02-g.
Calculations:
1. Determine the percentage of water in the hydrate
2. Determine the moles of the anhydrous salt and the moles of water.
3. Determine the mole ratio of salt to water.
4. Report your experimental formula for the hydrate.
Questions:
1. The method used in this experiment to find the experimental waters of hydration is not suitable
for all hydrates. Give at least two reasons why.
2. (A) Why is it necessary to let the crucible cool before measuring mass? (B) Why should the mass
of the crucible be measured immediately after the crucible cools, and not later?
3. Give some reasons why your percentage may differ from others in the class.
4. How would your experimental results be affected if you did not use a desiccator when cooling the
crucible and contents?
5. What is the formula for this hydrate Barium Chloride?