Determining the Concentration of an Unknown Solution
Problem:
Mr. Duty recently went to a principal’s conference where he was poisoned with “arsenic” in his drink (the other
principals were all jealous of the great students at Tahoma HS). Luckily, he was able to make a full recovery. Since Mr.
Duty did not finish his drink, CSI investigators were able to get a sample of the original poisoned “arsenic”. They also
went to the chemistry labs in the schools of the top 4 suspects and took samples of the “arsenic” in their labs. Luckily all
of these samples were labeled with their molarities (see table below). Your knowledge of solutions chemistry is needed
in order to determine the molarity of the solution that poisoned Mr. Duty so that the culprit can be nabbed. You will get
up to 50 mL of a known 1 M solution of “arsenic” and 10 mL of the unknown solution that poisoned Mr. Duty.
School
Enumclaw HS
Federal Way
HS
Bellevue HS
Kentlake HS
Molarity of
arsenic found
there
0.241 M
0.358 M
0.495 M
0.712 M
One of our thinking skills is Analysis – this is the breaking apart of a problem into parts to determine meaning.
You will use this skill to solve the problem of who poisoned Mr. Duty.
Step 1: Explain the question that will have to be answered in order to determine who poisoned Mr. Duty.
Step 2: Identify and explain the relevant pieces of information that you will have to determine in order to
answer your question in part 1. (Hint: You should be using Beer’s Law which compares absorbency and
concentration.) Then you need to determine the process for collecting this information. (Hint: You will need to
make a series of dilutions from your 50 mL of 1 M stock solution.) After you have completed step 2, call your
teacher over to hear your explanations.
Step 3: Collect and record your data using Logger Pro. You will have a handout that explains the procedures
for using logger pro and the colorimeters.
Step 4: Interpret the data that you collected using the colorimeter. Discuss how it supports the relationship
between absorbency and concentration.
Step 5: Write a statement that answers the question from part 1 and explain how your data and graph
supports this statement.
Procedure:
1) Label 5 clean test tubes 1-5. Following the set of dilutions you calculated, prepare each test tube according. Use one
pipette for H2O and one for the 1 M stock “arsenic”. When done, thoroughly mix each solution. Make sure to clean and
dry the stir rod between test tubes. In the 6th test tube obtain a sample of the unknown.
2) Prepare the computer for data collection by opening up Logger Pro from the desktop. From the Menu Bar, choose
File, Open, and then from the Chemistry with Venier folder choose Experiment 11 (Beer’s Law). If your computer does
not show an absorbance reading from the colorimeter, choose Experiment (from the Menu Bar), Connect Interface, Lab
Pro, and then COM1. If another screen pops up hit OK. Your screen should now show an absorbency reading.
3) Put your arrow on the x-axis (concentration) and drag it until it is scaled from 0 to 0.8 mol/L. Check that the y-axis
(absorbance) reads from 0 to 1.
4) You are now ready to calibrate to colorimeter. Prepare a blank by filling a cuvette ¾ full with distilled water. Some
things to keep in mind when using a cuvette:
- Always handle cuvettes by the top edge of the ribbed side
- The smooth side of the cuvette should always be wiped clean of all finger prints and not touched again.
- Always position the smooth side of the cuvette so that it aligns with the white line on the colorimeter.
THE NEXT STEPS ARE FOR THE OLD COLORIMETERS!!! If you have the newer ones skip ahead. Choose Calibrate from
the Experiment Menu and then click Calibrate Now. Place the blank cuvette in the cuvette slot of the colorimeter and
close the lid. Turn the wavelength knob of the colorimeter to the 0% T position. In this position the light source is turned
off, so no light is received by the photocell. Type “0” in the % edit box (%T). When the displayed voltage reading (middle
right) for Input 1 stabilizes click Keep.
For reading 2, turn the wavelength knob o f the colorimeter to the Red LED position. In this position the colorimeter is
calibrated to show 100% of the green light being transmitted through the blank cuvette. Type “100” in the % edit box
(%T). When the displayed voltage reading (middle right) for Input 1 stabilizes click Keep and then click OK. Then click
Done. The absorption reading should be very close to 0.000. DO NOT TOUCH THE KNOB ON THE COLORIMETER AFTER
THIS!!!
THESE ARE THE STEPS FOR CALIBRATING THE NEW COLORIMETER!! Press the < or > button on the colorimeter to select
the correct wavelength – for this experiment use 635 nm. Open the lid and put in the blank cuvette ¾ full with distilled
water. Close the door to the colorimeter and hold down the calibrate button until the red light begins to flash. The
absorption reading should be very close to 0.000.
5) You are now ready to collect absorbance data for the test tubes that you made by diluting the 1 M stock. Click Collect
(the play button). Empty the water from the cuvette. Using the solution in Test Tube 1, rinse the cuvette twice with the
solution and then fill it up ¾ full. (This is to ensure that the only liquid in the cuvette is from test tube 1.) Wipe the
smooth side of the cuvette and correctly place it in the colorimeter. After closing the lid, wait for the absorbance value
displayed on the monitor to stabilize. Then click Keep (the blue wheel on the top right), when prompted type the
concentration (molarity) of the sample you are measuring in the colorimeter, in the edit box and press the enter key.
The data pair you just collected should now be plotted on the graph. Put the contents of the cuvette back in Test Tube 1.
6) Repeat step 5 for each of your remaining test tubes. When prompted with the edit box, make sure you type in the
correct concentration for the test tube you are testing.
7) When all test tubes have plotted on your graph, print out one copy per group.
8) Rinse the cuvette twice with the unknown sample. Then fill it ¾ full with the unknown and put it in the colorimeter.
On the graph you have just printed, very clearly indicate the absorbance of the unknown and its corresponding point.
Determine which school was responsible for poisoning Mr. Duty and check your answer with your teacher.