A P CHEMISTRY
Lab 2-1 Determination of the Formula of a Hydrate
Pre-Lab Questions - (Must be completed before lab work begins.)
1. Suppose 2.3754 g of copper (II) sulfate pentahydrate is heated to drive off the water of crystallization.
Calculate what weight of anhydrous salt will remain.
2. In CuSO4. 5H2O, it was mentioned that four of the five water molecules held per formula unit of the salt
were attached by coordinate covalent bonds to the copper ion. The fifth water molecule is attached to the
sulfate ion, but by a different mechanism. Use your textbook or the internet to determine how a water
molecule might be bonded to a sulfate ion.
INTRODUCTION - Many hydrated substances lose their water of crystallization readily if the hydrate is
heated to a few hundred degrees. Usually the water of crystallization can be observed as steam escaping from
the crystals as they are heated or as a condensate on a cooler portion of the vessel used for the heating.
A common experiment in the characterization of new or unknown compounds is to measure the mass of
water driven off from a sample of the compound when it is heated. This is true whether the compound is a
hydrate or has merely adsorbed water on the surface of its crystals. For a true hydrate, it is often possible to
determine the molar ratio in which water is combined in the hydrated compound. Many hydrates, especially
those of transition metals, are stable and stoichiometric. That is, they contain a specific, definite amount of
water per mole of metal ion present.
In this experiment, you will be given a sample of an unknown hydrated metal salt. You will be told what ions
are present in the sample, but will not be told how many water molecules are bound in the compound per mole
of metal ion. By determining carefully the weight lost on heating a weighed sample of the unknown, you will be
able to calculate the percentage of water in the hydrated metal salt. From the percentage, you will be able to
determine the number of water molecules bound per metal ion in the unknown hydrate.
SAFETY:
 Wear safety glasses at all times while in the laboratory.
 Nitric acid (used for cleaning the crucible in this experiment) can burn the skin and can emit toxic fumes if
heated. If spilled, wash immediately and inform the instructor.
 When the crucible used in the experiment is heated, spattering of the contents may occur if too large a flame
is used initially. Heat very slowly at first with a small flame, and do not apply the full heat of the burner until
most of the water has been driven from the hydrate.
 The unknown metal hydrate may be toxic. Wash hands after handling and report any spills to the instructor.
Procedure
1. Record all data and observations directly in your notebook in ink.
2. Obtain a porcelain crucible and a cover that fits the crucible. These items are extremely fragile and
expensive.
3. If there is any solid material in the crucible, use a paper towel to wipe out the solid. Wash the crucible with
soap and water to clean out any remaining solids.
4. If the crucible cannot be cleaned with soap and water, transfer the crucible to the exhaust hood, and add a
few mL of 3 M nitric acid (Caution!). Allow the crucible to stand with the acid for 5-10 minutes. Discard
the nitric acid in the hood drain, and rinse the crucible with large amounts of water. If the crucible is still not
clean, consult with the instructor.
5. Set up a clay pipe triangle on a metal ring on a ring stand. Wipe the crucible with a paper towel, and then
place the crucible on the triangle. Set the cover onto the crucible so that it is slightly ajar. From this point
on, handle the crucible only with crucible tongs (when hot) or a paper towel (when cold), to prevent skin oils
A P CHEMISTRY
Lab 2-1 Determination of the Formula of a Hydrate
from adhering to the crucible and changing its mass.
6.
7.
8.
9.
Heat the crucible/cover with a small flame until all visible moisture has been removed from the crucible.
Increase the size of the flame, and heat the crucible/cover to red heat for 5 minutes.
Allow the crucible/cover to cool completely to room temperature.
Transfer the crucible and cover (using a paper towel to avoid oils from the fingers) to a watch glass. Weigh
the crucible and cover to the nearest mg (0.01 g).
10. Return the crucible/cover to the clay triangle, and reheat in the full heat of the burner for 5 more minutes.
11. Allow the crucible/cover to cool to room temperature and reweigh. If the second weight determination
differs from the first determination by more than 2 cg (0.02 g), reheat the crucible and cover for additional 5minute periods until this level of agreement is reached.
12. Obtain a sample of an unknown metal hydrate (caution) and record its identification number and appearance.
Place about 1 g of the hydrate sample into the crucible.
13. Reweigh the crucible, cover, and hydrate to the nearest cg (0.01 g). Place the crucible and its cover onto the
clay triangle, arrange the cover so that it is slightly ajar, and begin heating the crucible with a very small
flame.
14. Continue heating with a small flame for 5 minutes. Beware of spattering during this time period: if
spattering occurs, quickly push the cover over the crucible and remove the flame.
15. After you have heated the crucible gently, most of the water should have been driven from the crystals.
Increase the size of the flame somewhat, and heat the crucible and contents for 10 minutes at the higher heat
level with the cover slightly ajar. After the heating period, close the cover on the crucible, and allow the
assembly to cool to room temperature.
16. When the crucible has cooled completely to room temperature, weigh the crucible, cover, and contents to the
nearest cg (0.01 g).
17. Return the crucible to the clay triangle, and reheat the crucible with the cover slightly ajar for an additional
10-minute period. Allow the crucible to cool to room temperature and reweigh. If the second weight
determination differs from the first determination by more than 2 cg (0.02 g), reheat for an additional 10minute period, and reweigh.
18. From the weight determinations, calculate the percentage of water present in the hydrated salt. From the
information provided to you about what ions are present in the salt, determine the number of water
molecules per formula unit of the salt.
19. Dispose of the sample as directed by the instructor. Clean the crucible and cover, and reheat to constant
weight.
20. Use a second 1 g sample of your unknown hydrate to repeat the determination to increase accuracy.
A P CHEMISTRY
Lab 2-1 Determination of the Formula of a Hydrate
NAME _________________________________
PERIOD _____
ANSWER SHEET
Results / Observations
Unknown ID number
__________
Ions indicated to be present in the unknown
Color __________
Mass empty crucible/cover after first heating
____________________
Trial 1
__________
Trial 2
__________
Mass empty crucible/cover after second heating
__________
__________
Mass crucible/cover/sample before heating
__________
__________
Mass crucible/cover/sample after first heating
__________
__________
Mass crucible/cover/sample after second heating
__________
__________
1. Mass water driven off
__________
__________
2. % water in the hydrate
__________
__________
3. Mean % water in hydrate ____________________
4. Formula of the hydrated salt
____________________