AP Chemistry
The Determination of the
Percent Water in a Compound
The polarity of the water molecule, which makes it a great solvent for ionic compounds, causes water
molecules to cling to the structure of solid substances. When this occurs, the trapped water molecules are
called water of hydration and they become an integral part of the crystal structure.
There are many compounds that have a tendency to absorb water vapor from the air. These compounds
are said to be hygroscopic, and can be used as moisture-reducing agents. Other compounds absorb such
large quantities of water vapor that they will actually dissolve in their own water of hydration, a property
known as deliquescence.
In this experiment, you will test a hygroscopic ionic compound to determine its water of hydration.
Although the water molecules are securely attached to the ionic solid that you will test, they are
susceptible to removal by heat. You will gently heat a sample of the compound to drive off the water of
hydration. By measuring the mass of the sample before and after heating, you can determine the amount
of water in the sample and calculate its water of hydration.
OBJECTIVES
In this experiment, you will
 Carefully heat a measured sample of a hygroscopic ionic compound.
 Determine the water of hydration of the compound.
 Complete the chemical formula of the compound.
MATERIALS
crucible with cover
one of the following compounds:
crucible tongs
magnesium sulfate, MgSO4•nH2O
spatula
barium chloride, BaCl2•nH2O
ring stand, ring, and clay triangle
manganese (II) sulfate, MnSO4•nH2O
lab burner
sodium carbonate, Na2CO3•nH2O
desiccator
balance
PROCEDURE
Part 1 Preparation
1. Obtain and wear goggles and apron.
2. Place a clean, dry crucible with a cover in a pipestem triangle mounted on the iron ring.
Leave the cover slightly off so that the heating will drive off any water that remains in the crucible.
Adapted from Advanced Chemistry with Vernier, Zumdahl, World of Chemistry, 2002 and Heath and Company, 1987
2-1
Heat with Bunsen burner for two to three minutes to make sure that the crucible is dry.
See setup on the next page in Figure 1.
3. From this point on, you should not touch the crucible with your hands. Use only the crucible tongs.
Allow the crucible to cool for at least ten minutes, then measure the combined mass of the empty
crucible and its cover.
Record the mass in your Data Table.
4. Place enough of the hydrate that you are assigned into the crucible to that it is one-fourth to onethird
full. Use a spatula or similar implement to break up any large pieces of the substance by pressing the
pieces against the wall of the crucible. Measure and record the combined mass of the crucible, cover,
and hydrate.
Part 2
Evaporation of Water
1. Place the crucible, with the cover slightly off, on the pipestem triangle and begin heating.
2 - 2 Adapted from Advanced Chemistry with Vernier, Zumdahl, World of Chemistry, 2002 and Heath and Company,
1987
The Determination of the Percent Water in a Compound
Gradually increase the heat until the bottom of the crucible is a dull red. Maintain this temperature
for
five minutes.
2. Turn off the burner and place the crucible, with the cover, in a desiccator for cooling. Allow the
crucible to cool for about 15-20 minutes, then measure and record the mass of the crucible, cover,
and
contents.
3. Reheat the crucible for another five minutes to make sure that all of the water is driven off. Again,
cool it in the desiccator, then find and record the mass. If the masses that you determined in steps 2
and 3 do not agree within 0.03 g, repeat the heating and desiccating process one more time before
finding and recording a third mass.
Adapted from Advanced Chemistry with Vernier, Zumdahl, World of Chemistry, 2002 and Heath and Company, 1987
2-3
Cleaning Up
1. Carefully clean the crucible and dry it. Return it to its proper location.
2. Place the samples of hydrate in the waste containers on the reagent shelves between benches.
3. Clean up your lab station, returning all equipment to its proper location.
3. Wash your hands thoroughly before leaving the laboratory.
DATA TABLE
Compound selected for analysis
Mass of crucible and cover (g)
Mass of crucible, cover, and hydrated sample (g)
Mass of hydrated sample (g)
Mass of crucible, cover, and dehydrated sample – 1st weighing (g)
Mass of crucible, cover, and dehydrated sample – 2nd weighing (g)
Mass of crucible, cover, and dehydrated sample – 3rd weighing (g)
Mass of dehydrated sample (g)
Mass of water evolved (g)
DATA ANALYSIS
1.
What was the mass of the anhydrous salt?
2.
How many moles of the anhydrous salt did you have?
3.
How much water was lost from your salt?
4.
How many moles of water were lost from your salt?
5.
How many moles of water were there per mole of anhydrous salt?
2 - 4 Adapted from Advanced Chemistry with Vernier, Zumdahl, World of Chemistry, 2002 and Heath and Company,
1987
The Determination of the Percent Water in a Compound
6.
What is the formula for the hydrated salt?
CONCLUSIONS
1.
Can you suggest reasons why the procedure used in this experiment might not be appropriate
for all hydrates?
2.
A substance was found to have the following percentages:
Zinc 23%
Sulfur 11%
Oxygen 22%
Water 44%
What is the empirical formula of this compound?
3.
If a sample of 2.56 g of this substance were heated in a crucible as in this experiment, calculate
the mass of anhydrous compound that would remain in the crucible.
4.
Many different drying agents could have been in your desiccator. What characteristics should
these compounds have?
Adapted from Advanced Chemistry with Vernier, Zumdahl, World of Chemistry, 2002 and Heath and Company, 1987
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