Solubility of Compounds of Some Group II Elements
Aim
The aim of this experiment is to demonstrate the trends in solubility of the Group II
carbonates, sulphates(VI), suphates(IV) and hydroxides.
Introduction
In this experiment, you add each of the anion solutions to 1 cm3 of each cation
solution provided, drop by drop, util the first sign of a precipitate appears. For each
salt, the solubility is proportional to the number of drops of anion added.
Materials required (per student)
16 test-tubes, 4 test-tube racks, labels for test-tubes 5 teat pipettes (marked off at 1
cm3)
0.1 M solutions of the following cations: Mg2+, Ca2+, Sr2+, Ba2+.
1.0 M solution of OH-, 0.5 M solutions of SO42- and SO32- ions.
0.05 M solution of CO32- ions, distilled water
Technician’ notes
The 5 teat pipettes for each student may be replace by 8 burettes which are used to
serve the whole class. In this case, 2 burettes are for one kind of cation solution.
Procedure
1. Set up four rows of four test-tubes each.
2. For each row, label the first test-tube Mg2+, the second test-tube Ca2+, the third
test-tube Sr2+ and the fourth test-tube Ba2+.
3. Add 1 cm3 of the appropriate cation solution to each test-tube, using a teat pipette
with a 1 cm3 mark.
4. Label the first row of test-tubes OH-, the second row SO42-, the third row SO32-, and
the fourth row CO32-.
5. Add the OH-, drop by drop, with shaking, to each cation solution in the first row,
until the first sign of a precipitate appears.
6. Record the number of drops of OH- solution used in a copy of Results Table 4.
7. Repeat steps 5 and 6 with the remaining anions and cations.
8. If a precipitate appears suddenly, during the addition of a drop, then you should
classify the precipitate as slight(s) or heavy(h).
9. If no precipitate appears after forty drops, then write ‘40+’ and regard the salt as
soluble.
Results Table
Cation
Number of drops of anion solution added to give a precipitate
solution OHSO42SO32CO32Mg2+
Ca2+
Sr2+
Ba2+
Questions for discussion
1. For Group II, what are the trends in solubility of the salts listed below?
(a) hydroxides
(b) sulphates(VI)
(c) sulpates(VI)
(d) carbonates
2. Comment your results in the following aspects.
(a) Reliability
(b) Improvement of the experimental method
3. Give reasons that explain the group trend in solubility.
4. In the Introduction Section, it is stated that ‘… the solubility is proportional to the
number of drops of anion added.’ What assumption has been made?