MHS AP Chemistry
Determination of the Formula of a Hydrated Salt
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.
In this experiment, the number of water molecules associated with each
formula unit of 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
can be used to calculate the formula of the compound.)
Clean a crucible and cover with tap water and then rinse 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 1.0000 and l.5000 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. At this point you have
all of the information needed to determine the percent water in the sample
and the number of waters of hydration in the formula.
What is the name of the salt (not the hydrate part)?
Calculate the percent water in the hydrated sample. Be sure the report
the answer to the proper number of significant figures.
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.
Write the complete & correct formula for the hydrated compound you
started with. For examples, see the front of this sheet.
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
Calculations & Wrap-up
Each student should pass in a clean, organized data table for this
lab, and answer questions 1-6 above. Show all your data in a data
table, and show all your calculations with labels.
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