CHEM 203 City Colleges of Chicago Harry S Truman College Qualitative Analysis Cations Group I Lab 3

Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
Exp. 7 – Qualitative Analysis – Cations: Group I
Introduction
Qualitative analysis is a method used for identification of ions or compounds in a sample. In many cases,
qualitative analysis will also involve the separation of ions or compounds in a mixture. The separation of ions is
easily achieved by taking advantage of their solubility properties.
Qualitative Analysis Scheme for the Group I Cations
Upon adding HCl to a solution containing all common metal ions, a white precipitate forms. This precipitate
contains only AgCl(s), Hg2Cl2(s), and PbCl2(s) because only Ag+, Hg22+, and Pb2+ form insoluble salts with Cl-. These
three cations form the “Chloride Group”, which is Group I of the Quantitative Analysis Scheme.
Ag+ (aq) + Cl- (aq) → AgCl (s, white)
Hg22+ (aq) + 2Cl- (aq) → Hg2Cl2 (s, white)
Pb2+ (aq) + 2Cl- (aq) → PbCl2 (s, white)
After centrifuging the solution to collect the precipitate at the bottom of a test tube, the supernatant solution
is removed (decanted). By placing the supernatant solution into another test tube, the Ag+, Hg22+, and Pb2+ are
separated from all other metal ions that were in the solution. The Ag+, Hg22+, and Pb2+ reside in the original test
tube as a mixture of white solids.
To determine which of the three cations are present in the precipitate requires additional tests. While AgCl(s),
Hg2Cl2(s), and PbCl2(s) are relatively insoluble in cold water, PbCl2(s) is quite soluble in hot water. Therefore,
adding cold water to the precipitate and (subsequently) heating the solution dissolves the PbCl2(s), if any is
present. Centrifuging the hot test tube separates the Pb2+ (now in the supernatant solution if present) from the
AgCl(s) and Hg2Cl2(s) remaining as white precipitates at the bottom of the test tube. Decanting the supernatant
solution into another test tube and adding Na2CrO4(aq) produces a yellow precipitate of PbCrO4(s), if any Pb2+ is
present. The formation of PbCrO4(s, yellow) confirms the presence of Pb2+ in the original solution.
Pb2+(aq) + CrO42-(aq) → PbCrO4(s, yellow)
Treating the white precipitate in the original test tube with NH 3 (until the solution is basic) allows for the
separation of the remaining ions, Ag+ and Hg22+. The NH3 (a complexing agent) increases the solubility of the
AgCl(s).
AgCl (s, white) ⇄ Ag+ (aq) + Cl- (aq)
Ksp= 1.8×10-10
Ag+(aq) + 2NH3(aq) ⇄ Ag(NH3)2+(aq)
Kf = 1.7×107
After the addition of the NH3 and subsequent centrifugation, a dark precipitate will collect at the bottom of the
test tube if any Hg2Cl2(s) is present. Decanting the supernatant solution into another test tube separates the Ag+
(in the solution) from the Hg22+ (in the solid). Adding Na2CrO4(aq) to the supernatant solution produces a red
precipitate of Ag2CrO4(s), if any Ag+ is present. The formation of Ag2CrO4(s,red) confirms the presence of Ag+ in
the original solution.
Ag+(aq) + CrO42-(aq) → Ag2CrO4(s, red)
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
Treating the dark precipitate in the original test tube with Na2CrO4(aq) produces an orange precipitate of
Hg2CrO4(s), if any Hg22+ is present. The formation of Hg2CrO4(s, orange) confirms the presence of Hg22+ in the
original solution.
Hg22+(aq) + CrO42-(aq) → Hg2CrO4(s, orange)
Procedure
Simulation – Data Collection
1. Locate the simulation using the following link: http://web.mst.edu/~gbert/qual/qual.html
(Note: The discussion, flow sheet, and procedure has been modified from the online text presented as
part of the simulation. Use the instructions in this handout and video demonstration to complete this
laboratory exercise and report.)
2. The discussion, flow sheet, and procedure has been modified from the online text presented as part
of the simulation. Use the instructions in this handout and video demonstration to complete this
laboratory exercise and report.
3. Using this simulator you will perform a series of experiments (outlined in the modified flow chart
below) to determine the presence or absence of Ag+, Hg22+, and Pb2+ in unknown solutions.
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
Part 1 – Trial Unknown
1. Watch the “Exp. 7 – Qualitative Analysis DEMO Video” to become familiar with the simulation
features and the steps involved in determining the presence or absence of Ag+, Hg22+, and Pb2+
in a sample.
2. From the SAMPLE menu (upper left side), select Trial Unknown.
a. Left click on the bottle to activate the dropper.
b. Left click on the first test tube to designate the dropper’s destination.
c. Left click on the dropper to deliver 4 drops of the Trial Unknown.
d. Left click on the bottle to return the dropper.
3. Go to the REAGENTS menu (upper left side), select 3M Hydrochloric acid (HCl).
a. Left click on the bottle to activate the dropper.
b. Left click on the first test tube to designate the dropper’s destination.
c. Left click on the dropper to deliver 2 drops 3M HCl.
d. Left click on the bottle to return the dropper.
4. Select Centrifuge.
5. Select Decant.
a. Left click on the second test tube to designate the dropper’s destination.
b. Select Discard. Important Note: Do not discard any solutions unless instructed. All
solutions generated in upcoming steps will undergo additional tests.
6. Test tube 1 contains SOLID 1 (see the modified FLOW CHART on p. 5). Each segment of the
analysis method is labeled 1-5 in BLUE to the left or right of the modified FLOW CHART. A BLUE
bracket indicates the sequential processes of the respective segment.
a. Perform each process in the order listed for segment 1.
b. Perform each process in the order listed for segment 2.
c. Perform each process in the order listed for segment 3.
d. Perform each process in the order listed for segment 4.
e. Perform each process in the order listed for segment 5.
7. After performing the experiment (segments 1-5), select Check Unknown located upper center
column below the Title.
a. Select YES for the ions that were present in the Trial Unknown.
b. Select NO for the ions that were not present in the Trial Unknown.
c. Select Check Answer.
d. Your results will be “graded” instantly.
• If three blue checks appear, obtain an image (photo, screenshot) of the
graded results to include in your lab report.
• If less than three blue checks appear, repeat the experiment until you achieve
the correct results. Obtain an image (photo, screenshot) of the graded results
to include in your lab report. (Note: The program randomly generates the trial
unknown.)
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
Part 2 – Assigned Unknown
1. From the SAMPLE menu (upper left side), select Assigned Unknown.
a. Enter a number between 101 and 999. A number will be randomly generated, but
you may select your own. (Note: Do not “share” an Assigned Unknown number
unless you are working as partners, which must be indicated on the lab report at the
time of submission.)
b. Left click on the bottle to activate the dropper.
c. Left click on the first test tube to designate the dropper’s destination.
d. Left click on the dropper to deliver 4 drops of the Trial Unknown.
e. Left click on the bottle to return the dropper.
2. Go to the REAGENTS menu (upper left side), select 3M Hydrochloric acid (HCl).
a. Left click on the bottle to activate the dropper.
b. Left click on the first test tube to designate the dropper’s destination.
c. Left click on the dropper to deliver 2 drops 3M HCl.
d. Left click on the bottle to return the dropper.
3. Select Centrifuge.
4. Select Decant.
a. Left click on the second test tube to designate the dropper’s destination.
b. Select Discard. Important Note: Do not discard any solutions unless instructed. All
solutions generated in upcoming steps will undergo additional tests.
5. Test tube 1 contains SOLID 1 (see the modified FLOW CHART on p. 5). Each segment of the
analysis method is labeled 1-5 in BLUE to the left or right of the modified FLOW CHART. A BLUE
bracket indicates the sequential processes of the respective segment.
a. Perform each process in the order listed for segment 1.
b. Perform each process in the order listed for segment 2.
c. Perform each process in the order listed for segment 3.
d. Perform each process in the order listed for segment 4.
e. Perform each process in the order listed for segment 5.
6. After performing the experiment (segments 1-5), select Check Unknown located upper center
column below the Title.
a. Select YES for the ions that were present in the Assigned Unknown.
b. Select NO for the ions that were not present in the Assigned Unknown.
c. Select Prepare for Print. (Note: Your results
will NOT be “graded” instantly.)
d. Enter your first and last name and select OK.
e. A mini-report form will appear. Obtain an
image (photo, screenshot) of this form to
include in your lab report.
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
IMPORTANT – READ BEFORE STARTING THE EXPERIMENT
1. Each student will submit individual lab report.
2. The answers to all parts of the lab report can be typed or inserted into this Word document.
3. When completed, save as PDF file, and submit to Brightspace > Assignments > Lab 7, by indicated due
date.
4. File name should include student’s name and report submitted. Example “R. Todorovic-Lab 7”.
Pre-Lab Questions
1. In the Quantitative Analysis Scheme, there are five groups of metal ions. What ions are in Group I?
2. What do all the metal ions of Group I have in common? In other words, what makes them different from
all other metal ions?
3. How do the metal ions of Group I differ? In other words, what property allows them to be easily
separated from one another?
Data and Conclusions
Part 1 – Trial Unknown (Insert image)
Part 2 – Assigned Unknown (Insert image)
Post-Lab Questions
1. The solubility of one Group I ion increased substantially at a higher temperature.
a. Which ion was it?
b. Why was this difference in solubility important to the goal of the experiment?
2. The solubility of one Group I ion increased substantially after the addition of NH3.
a. Which ion was it?
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Exp. 7 – Qualitative Analysis – Cations: Group I
Chemistry 203 – General Chemistry II
Dept. of Physical Sciences & Engineering
Wilbur Wright College
b. Explain why the solubility of this ion increased when NH3 was added. Use LeChatlier’s Principle.
c. Why was this difference in solubility important to the goal of the experiment?
Student Name:
Date:
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