Chemistry iCAP#23: Qualitative Analysis Flowchart Practice
Name _____________________________________ Date _______ Block ________ GRN GOLD
Background:
In the qualitative analysis lab, you will ultimately be responsible for identifying a series of
unknown ion solutions. In order to be able to do this, you will first collect data from a group of
experiments involving these ions.
Lab Details:
The ions being tested are: Pb+2, Ag+1, Fe+3, and Ni+2. You will receive solutions that
contain the ions individually, and a solution that contains all 4 ions.
Basic Characteristics of Ions
Fill in the following chart with the formulas of the compounds formed when your 4 ions
react with other ions in solution. Indicate by circling them if they would result in the formation of
a precipitate.
Pb+2
Ag+1
Fe+3
Ni+2
Cl-1
CrO4-2
OH-1
The circled compounds in the table above will be those that will help you to determine
the identity of your unknown ions.
Creation of a “Scheme” or Flowchart
One of the first assignments during the lab (after preliminary tests have been completed)
will be the creation of a flowchart. This flowchart will help you to more clearly see how you can
narrow down the identity of your unknown ion solutions.
Using this sample information, create a flowchart that will “separate” the ions for
analysis.
Example:
Suppose a solution contains either of the ions A+2 or B+. It is known that
+2
A reacts with C- to produce a yellow precipitate, while B+ reacts with C- to form
a green precipitate.
Identifying which ion is present in solution only requires additions of C to the solution and observation of the resultant precipitate color. Difficulty arises
if the solution contains both ions, however. In this case, the combination might
result in a slightly lighter green precipitate, making it difficult to determine if the
solution contained only B+ or both ions. The presence of B+ interferes with the
test for A+ and vice versa.
The strategy for investigating solutions with multiple ions is to find
chemical reactions that will first separate the ions. Once the separation has been
accomplished, the ions can be tested and identified individually. For example, in
the case of the A+2 and B+ mixture, it is found that the addition of base will result
in the formation of A(OH) 2 which is insoluble and precipitates from solution. In
contrast, B+ does not react with the base and remains as an ion in solution.
Therefore, treating the unknown solution with sodium hydroxide can
result in one of the following two situations:
1) No precipitate. The conclusions is that A+2 is not present. Confirm
the presence or absence of B+ by adding C-1 and looking for a green
precipitate.
2) Presence of precipitate.
Separate the precipitate (possible
containing A+2) from the supernatant (possible containing B +) by
centrifuging the test tube and decanting (pouring off) the
supernatant. The supernatant is then tested for the presence of B +
as described above. The precipitate should also be tested to verify
that it contains A+2 and not some other impurity that precipitates in
the base. To do this, dissolve the precipitate in neutral or slightly
acidic solution, add C-1, and look for the yellow precipitate.
Qualitative Analysis Flowchart for separation and identification of A+2 and B+
Unknown solution of metal ions, A+2 and B+1