The Formula of a Hydrate
Pairs are preferred but you may work alone if you wish. Groups of three or
more are not permitted.
The Report
One report per person. Use standard headings for lab reports. Show all data and calculations, and
lay them out neatly and attractively. State how many waters of hydration are present for every
formula unit of cobalt(II) chloride, and the chemical formula for the hydrate.
The Problem
When a solution of CoCl2 is evaporated, some water is retained in the crystal even though the
powder appears to be dry. Solid compounds like this, which contain water as part of their crystal
structure, are called hydrates. It is possible to remove the water through heating. The product
obtained after driving off waters of crystallization is called the anhydrous form of the compound.
Your task is to determine the number of water molecules in the formula of the
cobalt (II) chloride hydrate. By heating a sample of the compound until all the waters of
crystallization have been driven off, and carefully noting the changes in mass of the powder, you
will be able to determine which of the following formulas is correct:
CoCl2·4H2O
CoCl2·5H2O
CoCl2·6H2O
CoCl2·7H2O
Reagents and Equipment
5 g cobalt chloride hydrate powder
ceramic crucible
ring stand, ring, and pipe stem triangle
tongs
Bunsen burner
Data
mass of empty crucible _________ g
mass of crucible and cobalt (II) chloride hydrate ________ g
mass after heating _________ g
mass after second heating _________ g
colour of cobalt chloride before heating ____________
Procedure
1. Find the mass of a ceramic crucible. Record all masses to two decimal places.
2. Add approximately 4 - 6 grams of cobalt (II) chloride to the crucible, and record the total
mass. Record all masses to two decimal places.
“The Formula of a Hydrate”
Revised November 5, 2009
Continued over …
3. Heat the cobalt(II) chloride hydrate until all the water has been driven off. There will be a
colour change from dark to light blue. Don’t overheat. If the colour begins to turn black,
then reduce heat or stop altogether. When heating is done, allow the crucible to cool.
4. Once the crucible is cool enough to touch with the back of the hand, carry it with tongs to
the balance and measure its mass.
5.
Heat again for several minutes, and find the mass again. If the mass has changed less
than 0.02 g, record this mass and discontinue heating. If the mass has changed by more
than this amount, then heat again.
6. Let the crucible cool until it is room temperature. Wash the anhydrous cobalt((II) chloride
into the container provided (the water will be evaporated and the crystals used again.) Note
the colour change that occurs when the anhydrous cobalt chloride contacts water again.
CAUTION: Do not add water to a hot crucible. The crucible will crack and become useless.
Calculations
1. Calculate the mass of water removed.
2. Calculate the mass of anhydrous cobalt (II) chloride that remained in the
crucible after heating.
3. Calculate the number of moles of water that was removed.
4. Calculate the number of moles of anhydrous cobalt(II) chloride that were present.
5. Calculate the ratio of moles of water to moles of anhydrous cobalt(II) chloride.
n H2O
ratio = ---------n CoCl2
6. Round the ratio in Step 5 to the nearest whole number. This value represents the number
of waters of hydration.
7. Write the formula of cobalt(II) chloride hydrate showing the numbers of waters of
hydration. It will be one of the four possible formulas mentioned in the introduction.