Qualitative Analysis to Identify Unknown Ionic Compounds
If an unknown sample is one of a limited number of possible compounds, a simple test often
can be used to determine its composition. For example, a flame test can distinguish between KCl and
NaNO3. A drop of potassium ferricyanide solution can be used to distinguish between FeCl2 and
FeCl3. There are hundreds of reactions that are useful in discriminating between a few possible
compounds. Qualitative analysis is very important to the forensic chemist who is interested in solving
crimes. Certainly, a drop of hydrochloric acid would be enough for the police department’s forensic
chemist to distinguish between a bag of cocaine and a bag of baking soda.
In this experiment you will be asked to identify the contents of a number of vials. The
substance in each vial is one of the two compounds as noted in the list below. To decide which
compound is present you will need to use some of the tests that you learned in previous labs (See
Flame Tests Lab, Iron Tests Lab, Qualitative Tests for Sulfite, etc. Lab, and Double Replacement
Reactions Lab). For each unknown you need to perform two tests to identify the substance. For
instance, a sample containing either NaCl or KNO3 could be identified using a flame test since K and
Na ions give different colored flames. A second confirming test would be to dissolve the unknown in
distilled water then add a few drops of aqueous AgNO3. If the unknown were KNO3, we would expect
no precipitate to form since all species are soluble. However if NaCl is present, there should be an
obvious precipitate of AgCl formed.
Prelab assignment: Enter lab title and purpose in your lab book. On the left hand page, paste in the
list of possible unknowns and briefly note some tests which will be useful for identification.
Unknown Identification:
Use your results from the iron lab, flame test lab, precipitates lab and qualitative tests lab to
design a procedure to determine which of the two possible substances is present in each of the 10 vials.
You must have 2 tests for each vial. In designing a procedure, be sure to consider all four ions
potentially present. A useful test should give a different result with each of the compounds. You
should be able to predict two possible outcomes for each test before you begin. To prepare the
unknowns for testing, simply dissolve a few crystals in a few drops of distilled water in your spot plate.
Unknown Vial Number
1
2
3
4
5
6
7
8
9
10
Possible Compounds Present
KNO3
NH4Cl
FeCl2
Na2SO3
LiNO3
Sr(NO3)2
(NH4)2CO3
LiCl
Na2SO3
Fe(NO3)3
or
or
or
or
or
or
or
or
or
or
NaCl
MgCO3
FeCl3
KCl
Na2CO3
Na2SO3
ZnSO4
NaNO3
Li2CO3
Zn(NO3)2
Qualitative Analysis to Identify Unknown Ionic Compounds (cont’d)
Lab Report
Purpose: To use appropriate lab tests to identify which of two possible unknowns is present in each
sample.
Procedure: You do not need to rewrite procedures that are already pasted in your lab book elsewhere.
Simply write “See page ## for Iron Test procedure” etc. for each of the tests you used in the lab.
Data : Raw data, in ink, should be recorded in your lab book DURING LAB!!
Data and Results Table : The data table for the lab should include
 Each pair of possible compounds
 Name or description of each test performed (Precipitate tests must specify what you are
combining the unknown with. e.g. “Precipitate test with AgNO3” or “Iron test with
KSCN”
 Observations for each test (colors, fizzing, etc)
 Result for each test (which compound is indicated by observations?)
Conclusion: No written conclusion required. The above info in your lab book will be considered
as part of your final exam grade.