ACADEMIC CHEMISTRY
PERIOD:
NAME:
DATE:
PERCENT COMPOSITION of A HYDRATE LAB
INTRODUCTION
When many ionic compounds are crystallized from a water solution, they
include individual water molecules as part of their crystalline structure. Such
crystals are known as hydrates. If the substances are heated, this water of
crystallization may be driven off and leaves behind the pure anhydrous crystal.
Because the law of definite proportions (or definite composition) also applies to
crystalline hydrates, the number of moles of water driven off per mole of
anhydrous compound should be a simple whole- number ratio. This ratio can be
used to find the formula of the hydrate.
The formula for a hydrated crystal includes the formula for the anhydrous
crystal  n H2O, where n is the number of moles of water (or molecules of water)
in the crystal. For example, Epsom salts has the formula MgSO 4  7 H2O. When
determining the molar mass of the crystal, the mass of 7 water molecules should
be added to the total mass of the magnesium sulfate. The names of hydrated
crystals include the name of the anhydrous ionic compound followed by
_____________hydrate, where the ______ is a prefix that shows the number of
water molecules included in the formula. MgSO4  7 H2O is named magnesium
sulfate heptahydrate.
In this lab, we will determine the formula for the hydrate of CuSO 4 by
heating the crystal to dryness, finding the moles of water driven off and
calculating the mole ratio of water to the anhydrous crystal. In the end, we will
compare to the theoretical hydrate formula given on the chemical bottle.
PRELAB
Answer the following questions using complete sentences on a separate sheet of
paper.
1. Define anhydrous crystal.
2. Determine the molar mass of H2O by adding together the total masses of
all the elements in the compound.
3. Determine the molar mass of copper (II) sulfate by adding together the
total masses of all the elements in the compound.
4. What is the name of the equipment that will hold the hydrate while it is
being heated?
MATERIALS
Balance
Bunsen burner
crucible and lid
crucible tongs
CUSO4, hydrated
distilled water
ring stand
ring
clay triangle
stirring rod
weighing dish
dropper pipet
ACADEMIC CHEMISTRY
PERIOD:
NAME:
DATE:
PROCEDURE
1. Put on goggles.
2. clean your crucible, lid and crucible tongs. Place the crucible and lid on
the clay triangle with the lid slightly tipped. Heat the crucible and lid until it
has a slight red glow. Turn off your burner and allow the crucible and lid
to cool for 5 minutes. From this point on, do not touch the crucible and lid
with your hands.
3. Determine the mass of the crucible and cover to the nearest 0.01 g and
record the mass in your data table.
4. Using a spatula, add approximately 2 grams of copper sulfate hydrated
crystal to the crucible. Break up any large crystals before placing them in
the crucible. Determine the mass of the crucible, lid and hydrate and
record in the data table. Also record some qualitative observations of the
crystal.
5. Place the crucible on the triangle and position the cover so there is a small
opening. Heat the crucible very gently to avoid spattering of the crystal.
Gradually increase the temperature and heat the crucible for a total of 5
minutes. Record any qualitative observations. A color change is normal,
but if you have heated too quickly, the hydrate will turn yellow, which
indicates that it has been decomposed.
6. Allow the crucible to cool for 5 minutes.
7. Find the new mass of the crucible, lid and anhydrous crystal. Record in
the data table.
8. Heat the covered crucible and contents for another 5 minutes. Allow it to
cool, find the mass and record in the data table. If this new mass and the
previous mass differ by less than 0.01 g, you may assume that all the
water has been driven off. Otherwise, repeat the process until the mass
no longer changes, which indicates that all of the water has evaporated.
Record the mass after the final heating as the constant mass of sample,
crucible, and lid in your data table.
9. Using a dropper pipet, put a few drops of water onto the anhydrous
sample and record observations of what happens.
10. Clean all apparatus, dry and return to the proper location. Wipe your lab
bench and wash your hands before starting any calculations.
ACADEMIC CHEMISTRY
PERIOD:
NAME:
DATE:
DATA
Qualitative observations of crystal before heating:
Qualitative observations of crystal after first and second heating:
Qualitative observations of crystal after adding water:
Values recorded in lab:
Mass of empty crucible
and lid
Initial mass of crystal,
crucible and lid
Mass of sample, crystal
and lid after first heating
Mass of sample, crystal
and lid after second
heating
Constant mass of sample,
crucible and cover
Theoretical formula of the
hydrated crystal (see
chemical jar).
Value and unit
33.25 g
34.22 g
33.90 g
33.91 g
33.91 g
CuSO4
· 5 H2O
ACADEMIC CHEMISTRY
PERIOD:
NAME:
DATE:
CALCULATIONS
Calculated values:
Initial mass of hydrated
crystal
Mass of anhydrous
crystal (constant mass)
Difference in mass of
crystal (hydrated –
anhydrous)=Mass of
water driven off
Molar mass of water
Moles of water driven
off (start with g of H2O
driven off)
Molar mass of CuSO4
Moles of anhydrous
crystal (start with g of
anhydrous crystal)
Mole ratio of H2O to
CuSO4
Experimental formula
of the hydrated crystal
% error – compare the
experimental formula
to the theoretical
formula of the hydrate.
Calculation – show work here including units
and sig figs.
Value and
unit
ACADEMIC CHEMISTRY
PERIOD:
NAME:
DATE:
SUMMARY
HINT: In this lab, we determined the formula of the copper sulfate hydrate to be
________________________. When compared to the theoretical value of
_________________, we had a __________% error. The errors that occurred in
lab include (3) ____________________ and they affected our overall result by
__________________________________________. Followed by an overall
statement of the success of the lab.
POST LAB QUESTIONS
1. Name the hydrated crystal used in this lab.
2. Discuss how the following errors may affect your masses recorded in the
lab.
a. Touching the crucible with your hands.
b. Weighing the crucible while it was still hot.
3. Write the formula for barium chloride dihydrate.
4. Name the following hydrate: Na2S  9 H2O.
5. Some cracker tins include a glass vial of drying material in the lid. This is
often a mixture of magnesium sulfate and cobalt chloride. As the mixture
absorbs moisture to form hydrated compounds, the cobalt chloride
changes from blue-violet to pink. The blue compound is a dihydrate and
the pink is a hexahydrate. Write formulas for the two cobalt (II) chloride
hydrates. How could you change the hexahydrate back to the dihydrate?
6. A 5.0 gram sample of Cu(NO3)2  n H2O is heated and 3.9 grams of the
anhydrous salt remains. What is the value of n?
7. Name the hydrated crystal in the last problem.