Chemistry
Chapter 6
DETERMINATION OF THE FORMULA OF COPPER (II)
SULFATE
Purpose
To determine the number of moles of water in a mole of hydrated copper (II) sulfate is
written as CuSO4•n H2O.
Introduction
Vocabulary: Salt, anhydrous salt, hydrated salt, crystal
A hydrate is a compound that contains water in its crystal structure. The water may be
removed from the salt in the laboratory by heating the salt. The salt without the water is
called an anhydrous salt. Here are some examples of hydrated salts:
CaSO4 · 2H2O CoCl2 6 H2O
MgSO4 · 7H2O
In a hydrate the water molecules are a distinct part of the compound but are joined to the
salt by connections that are weaker than the connections in the salt or the connections in
the water molecules. Notice we use a dot to connect the water units to the salt formula.
In this experiment you will measure the mass of copper(II) sulfate, then remove the water
from the crystals and measure the mass of the anhydrous salt. The data gathered will
allow you to determine the percent water in the hydrated salt and also the empirical
formula for the hydrated salt (McDougal Litel 2007).
Prelab question—see page 3
Materials
Bunsen burner, hydrated copper (II) sulfate, test tube, test tube holders
Procedure
1. Weigh a clean dry test tube.
2. Put about 1cm depth of copper (II) sulfate in the test tube and reweigh.
3. Record these weightings and those which follow in a table.
4. Now, use the Bunsen to heat the test tube gently for a minute and then strongly
for about 3 minutes. Hold the test tube with a test tube holder. Allow the tube to
cool and then reweigh it.
5. Heat the test tube again for about 2 minutes. Allow it to cool and then reweigh.
6. If the last two readings differ by more than 0.05g, repeat the heating and weighing
again.
Data
Observations: Record all observations including appearance of the salt and what happens
during heating.
Record your measurements with uncertainty and units. Give this table a title.
Mass of test tube:
Mass of test tube and hydrated copper (II) sulfate:
Mass of test tube and anhydrous copper (II) suflate:
Calculations: Show your work, and then write them in a table. Give this table a title, and
include units in calculations.
Mass of anhydrous copper (II) sulfate:
Moles of anhydrous copper (II) sulfate:
Mass of H2O:
Moles of H2O:
Mole ratio of H2O and CuSO4 = Moles H2O / moles CuSO4
Calculated empirical formula of CuSO4
Literature value of moles H2O
Percent error
*Mass of hydrated copper (II) sulfate: = (Mass of test tube and hydrated copper (II)
sulfate – mass of test tube)
Percent error 
Literature value Experiment al value
 100%
Literature value
Conclusions
1. Why is the salt heated again if you don’t get the same mass?
2. Write a conclusion and evaluate the experiment.
3. What are some sources of error for the experiment? If you made a mistake or had
bad lab technique, this is called “human error.” Think of other sources of error,
such as weaknesses in the procedure and with the equipment
Evaluations
If you had less than 10% error, then your lab technique was probably good. The rest of
the error is due to “random error” caused by limitations of the procedure and the
equipment.
1. How was your lab technique? Do you think you need to improve your technique?
2. How could you improve the procedure?
3. Was the equipment accurate enough? Do you think you need better lab
equipment?
Prelaboratory Question
Prelab question: The following data was collected in an experiment. Calculate the
formula of the hydrated salt.
Table 1: Mass in grams of heating hydrated copper (II) chloride
Mass of test tube:
22.88 g ± 0.01 g
Mass of test tube and hydrated copper (II) chloride:
24.17g ± 0.01 g
Mass of test tube and anhydrous copper (II) chloride:
23.90 g ± 0.01 g
A. Mass of anhydrous copper (II) chloride:
B. Moles of anhydrous copper (II) chloride:
C. Mass of H2O:
D. Moles of H2O:
E. Mole ratio of H2O and CuCl2= Moles H2O / moles CuCl2
F. Calculated empirical formula of CuCl2
G. Literature value of moles H2O
H. Percent error