Empirical Formulas - Unknowns
Each lab group will be given an unknown binary compound to analyze. The general formula of
each group's compound will be AxBy. Heating any of the compounds will cause them to
decompose according to the following equation:
AxBy  x A(s) + y B(g)
Since ``B'' is released as a gas, strong heating will leave only the ``A'' of the unknown in the
crucible.
Each lab group will be told their unknown number. Be sure to record the unknown number in
your report.
The gram atomic masses of ``A'' and ``B'' can be determined from the table below.
Unknown Number
Gram Atomic Mass
of ``A''
Gram Atomic Mass
of ``B''
1
107
55.5
2
195
11.2
3
238
61.8
4
104
18.0
5
126
65.4
6
112
4.84
7
141
48.8
8
154
13.3
9
219
12.6
10
73.0
25.2
11
84.0
9.68
12
119
82.4
13
224
19.4
14
255
29.4
Safety and Environmental Concerns:
*Be sure to wear safety goggles or other eye protection as directed by your teacher.
*The solids you will be working with in this experiment are safe if handled properly. The solids
are toxic by ingestion. If any of the solid comes in contact with your skin, wash with soap and
water generously to avoid any possibility of ingestion.
*Use caution in handling the hot crucible and its support in order to avoid burns. Report any
Empirical Formulas - Unknowns
Each lab group will be given an unknown binary compound to analyze. The general formula of
each group's compound will be AxBy. Heating any of the compounds will cause them to
decompose according to the following equation:
AxBy  x A(s) + y B(g)
Since ``B'' is released as a gas, strong heating will leave only the ``A'' of the unknown in the
crucible.
Procedure:
1.Clean a porcelain crucible with soap and water. Dry the crucible by placing the crucible on a clay triangle over a
laboratory burner and heat until red hot.
2.Carefully remove the crucible and cover with tongs and allow them to cool on the lab table.
3.When the crucible is cool, measure the mass of the crucible to the nearest 0.001 g. Record.
4.Add about 3 grams of the unknown to the crucible and measure the mass to the nearest 0.001 g. Record the mass.
CAUTION: The unknowns are poisonous and can be absorbed directly through the skin. Wash with water
immediately if unknown comes in contact with your skin.
5.Begin heating slowly. Increase the heat until you have heated the crucible strongly for about 5 to 8 minutes. You
are driving off all the ``B'' in your unknown in this step.
6.Remove the crucible from the triangle support, let it cool on the lab table, and measure the mass of the crucible and
contents (now just the ``A'' in your unknown). Record.
7.Reheat with a hot flame for a couple of minutes, cool, and measure the mass again. If the mass does not agree with
the mass measured in step 6, repeat until you have heated to a constant mass.
Data :
1.Prepare a data table to include the following information:
unknown number
mass of crucible
mass of crucible and your unknown AxBy ______
mass of your unknown
mass of crucible and the ``A'' in your unknown 1st time
mass of crucible and the ``A'' in your unknown 2nd time
____
____
____
____
____
Calculations:
1. What is the mass of ``A'' in your unknown
2. What are moles of ``A'' in your unknown (use information about your unknown from the table)
3. What is the mass of ``B'' in your unknown
4. What are the moles of ``B'' in your unknown
5.Determine the empirical formula of your unknown from the ``moles of A'' and ``moles of B'' calculated in number
1.
6. Calculate the precent error. ( Options see teachers notes)
Teacher's Notes for ``Empirical Formulas - Unknowns''
* I have the students do a percent error based on the percent comp. of water in barium chloride
di-hydrate. ( I don’t let the students know that all the compounds are the same until the post lab)
This sheet is not for distribution to the students.
All students are issued pure BaCl22H2O as their unknowns although they do not know that this
is the case. This experiment is clever because a wide range of formulas are obtained even though
all of the unknowns are identical..
Expected results are given in the table below.
Unknown #
Formula
Unknown #
Formula
1
A3B
8
AB2
2
AB3
9
AB3
3
A3B2
10
A2B
4
AB
11
A2B3
5
A3B
12
A4B
6
AB4
13
AB2
7
A2B
14
A2B3
This experiment is appropriate for first-year chemistry students.
Equipment and Chemicals Needed
Equipment:
ceramic crucibles
crucible tongs
clay triangles
Bunsen burners
electronic top-loader mg balance
Chemicals:
vials of BaCl2 2H2O
See laboratory instruction sheet for atomic masses of A and B.
Safety Concerns
See ``Safety and Environmental Concerns'' in the student handout.
Barium chloride is toxic by ingestion. The students should be instructed to observe standard
safety precautions to avoid ingestion.