Water of Hydration
Chemistry Lab
Name___________________________________ Date_____________________ Period_____
Introduction
Most ionic compounds easily dissolve in water. If the water is allowed to evaporate, the
ionic solid left behind will often take up a specific amount of water from the solution. A compound
that contains a set amount of water as part of its crystal structure is called a hydrate. This water
of hydration can be driven off by heating the hydrated substance; thus converting hydrate to its
anhydrous form. The anhydrous form can be referred to as the anhydrous compound or the
anhydrous salt. The number of moles of water driven off per mole of the anhydrous compound is
always a simple whole number. If the formula of the anhydrous compound is known, you can use
your data to determine the formula of the hydrate.
In this lab you will be driving off the water of hydration from a hydrate, then calculating
the number of moles of water released and finally using that information to determine the
empirical formula of the hydrate. Each group will be working with one of four different hydrates.
Materials
• electronic balance
• Bunsen burner
• crucible and cover
• ring clamp
• one of 4 hydrated compounds
• clay triangle
• ring stand
• matches
• microspatula
• tongs
• goggles
Procedure
1) Put on your goggles and tie back long hair.
2) Find a Bunsen burning a make sure it has provides a strong flame before proceeding.
3) Throughout the experiment, handle the crucible and cover with clean crucible tongs only.
Place the crucible and cover on the triangle as shown in Figure A. Position the cover
slightly tipped, leaving only a small opening for any gases to escape. Preheat the
crucible and cover for about 5 minutes. CAUTION: The crucible and cover are very
hot after each heating. Remember to handle them only with tongs. Do not place
the hot crucible and cover on any surface other than the lab bench.
4) Allow the crucible and cover to cool for 5 minutes, then use the tongs to transfer the
crucible and cover to the balance. Record the mass to the nearest 0.01 g.
5) Using a microspatula, add approximately 5 g of your assigned hydrate (RECORD THE
FORMULA IN YOUR DATA TABLE) to the crucible. Determine the mass of the covered
crucible and hydrate to the nearest 0.01 g and record.
6) Place the crucible with the hydrate on the triangle, and again position the cover so that
there is a small opening. Too large an opening may allow the hydrate to spatter out of
the crucible. Heat the crucible gently with a low flame to avoid spattering any of the
hydrate. Increase the temperature gradually for 2 or 3 minutes. Then heat strongly for at
least 5 minutes. (Heating can be increased by either increasing the Bunsen burner flame
OR lowering the clay triangle so that the crucible is closer to the flame. Be careful, the
ring clamp will be hot.)
7) Allow the crucible and cover to cool for 5 minutes and then (using the same balance as
before) determine their mass and record.
8) Again heat the covered crucible and contents strongly for 5 minutes. Cool for 5 minutes
and then determine the mass once again. If the last two mass measurements differ by
no more than 0.01 g, you may assume that all the water has been driven off. Otherwise,
repeat the heating process until the mass no longer changes.
9) Dispose of the hydrate as directed. Clean all apparatus and your lab station, and return
equipment to its proper place.
Data
Read through the procedure to determine what data you will be recording. Use a ruler to make a
data table. Record your observations in the data table.
Analysis Questions
1) Write the correct formula for your anhydrous salt.
2) Use your data to perform the following calculations:
a. the mass of the hydrate
b. the mass of the anhydrous salt
c. the mass of the water of hydration
d. the moles of the anhydrous salt
e. the moles of the anhydrous salt
f. the moles of the water of hydration
g. the whole number mole ratio of anhydrous salt to water.
3) Write the formula for the hydrate based on the calculations.
4) Write the correct name of your hydrate.
5) Determine the mass percent of water in your hydrate.
6) Obtain the correct formula for your hydrate. Determine the true mass percent of water in
the hydrate.
7) Use your answers from 5 (experimental value) and 7 (true value) to calculate the percent
error.
8) What might be the cause of your error?
9) Explain what a hydrate is.