Determination of Water in a Hydrate
A number of ionic compounds contain one or more waters of hydration
in their formulas. A good example of this is copper(II) sulfate which
exists in an anhydrous form, CuSO4(s),
as well as a pentahydrate form, CuSO4•5H2O.
Many anhydrous compounds have a strong tendency to absorb water vapor
from the air, thus becoming hydrated compounds. These anhydrous compounds
find use as moisture reducing agents. You may have noticed that containers
of such compounds are often found in bottles containing pills that would
decompose if moisture were present. Such compounds are said to be
Some of these compounds absorb water to such an extent that they actually
dissolve in the water that they take up. When this is the case, the
compounds are said to be deliquescent. Sodium hydroxide is
an example of this type of compound. On the other hand, some hydrated
compounds tend to spontaneously loose their water of hydration when they
are placed in a dry environment. These compounds are said to be efflorescent.
Sodium sulfate decahydrate, Na2SO4•10H2O
is an example of this type of compound.
In this experiment, the amount of water associated with a salt will be determined.
The amount of water in the hydrated compound will be determined by heating a
massed sample of the compound in order to drive off the water. From the
mass change, the percent water in the sample will be determined. [This
information will be used to calculate the formula of the compound.]
Record the name of the salt you are analyzing.
Use tap water to clean a crucible/cover, then rinse them with distilled
water. Place the crucible with cover on the clay triangle.
Make sure the cover is slightly askew so that water can escape when the
crucible is heated.
After receiving directions from your instructor, heat the crucible and
cover using a Bunsen burner for about five minutes. Following this
allow the crucible to cool for at least five minutes. When the crucible
is near room temperature, determine its mass.
Place between 1.000 and l.500 g of the hydrated compound in the crucible
and again determine the mass.
Place the crucible with sample and cover on the clay triangle and heat
for ten minutes. Make sure that the cover is askew. Allow the
crucible and contents to cool for about five minutes and then re-determine
the mass. Repeat this step until the change in mass is less than
0.050 grams. This will ensure that all the water is driven completely
out of the compound.
Determine the percent water in the sample [and the number of waters of
hydration in the formula]. Part of your grade will be based on how
accurate your results are.
Calculations & Questions
[Percent Water in a Hydrate score sheet][MHS
What is the name of your anhydrous salt?
Calculate the percent water in the hydrated sample. Be sure to report
the answer with the proper number of decimals.
[Optional: Calculate the moles of water and the moles of anhydrous
compound in your sample. Calculate the number of waters of hydration
in the formula from this information. Water is 18 grams per mole
and _________________ is __________ grams per mole.]
What effect would heating the hydrated sample for too short a time have
on the calculated percent water? Would the calculated percent water
be lower or higher than the actual one? Explain your answer.
Suppose the crucible and cover were not heated to dryness after being rinsed
with distilled water. Would the resulting determination of the percent
water in the hydrate be lower or higher than the actual one? Explain