Incorporating Engineering in High School Science and Math
Using Beer’s Law to Determine The Concentration of a Solution
Engineering Connection:
Chemical and Environmental Engineers are involved in environmental decisions
such as assessing the process of water purification. In this experiment your task will be
to determine the efficiency of a water treatment plant in removing a specific contaminant.
Identifying the Problem:
Your department at The Water Treatment Facility is in charge of determining
whether cobalt is being removed from the water. You are asked to measure
the absorbance of samples taken from various locations in The Water Treatment Facility.
Equipment/Materials:
0.15 M solution of CoCl2
CoCl2 soln (unknown concentration)
6 cuvettes
stirring rod
Jenway Spec
test tubes
10 mL graduated cylinder
pipet
ruler
deionized water
Safety:
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Goggles should be worn at all times in the laboratory.
Cobalt (II) chloride is toxic by ingestion.
Gloves should be worn if open wounds are present on the experimenter’s hands.
Use a safety bulb for pipetting. Never pipet by mouth.
Procedure:
Part I: Validating the Beer-Lambert Law
1. Prepare a series of CoCl2 standard solutions according to the following table.
Using a pipet, add the indicated volume of CoCl2 and the indicated volume of
deionized water. Mix the contents of each cuvette using a clean, dry stirring rod.
NOTE: Tube 1 was already prepared in Step 3, Part I.
Volume of 0.150 M CoCl2 sol.
(mL)
Volume of water (mL)
Tube
1
5.0
Tube
2
4.0
Tube
3
3.0
Tube
4
2.0
Tube
5
1.0
0
1.0
2.0
3.0
4.0
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2.
Place 5 cuvettes on a card labeled 1-5. Fill each cuvette about ¾ full with a
solution from one of the test tubes.
3. Measure and record the absorbance in Data Table 1. of each of the series of
solutions at the wavelength of maximum absorbance (500 nm).
4. Calculate the concentration of each of the solutions based on the dilution factor.
a. Example: Tube 2 = 4/5 x 0.150M = 0.120M
5. Graph absorbance vs. [Co2+]. Use a ruler to draw the best straight line through
the origin and as close as possible to all of the experimental points.
Part II: Determining the concentration of CoCl2 in a water treatment facility
6. Obtain about 5 mL of the 3 samples of CoCl2 solution of unknown concentration.
Sample A represents untreated water,
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Incorporating Engineering in High School Science and Math
Sample B represents water obtained after coagulation and sedimentation.
Sample C represents water obtained after the treatment cycle is completed.
7.
Place each solution in a cuvette and put it in the sample compartment of the Spec.
8.
Record its absorbance at the wavelength of maximum absorbance which was
determined in Part 1. Record this number in the Data Table.
9. Calculate the concentration using Beer’s Law and the data you gathered. (An
alternative method for finding the unknown’s concentration is to read it from the
graph that you constructed. Find the absorbance value for the unknown on the
“y” axis of your graph. Follow this point across until you intersect the line
(calibration curve), then go straight down to the “x” axis to find the
concentration.)
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Incorporating Engineering in High School Science and Math
USING BEER’S LAW TO DETERMINE
THE CONCENTRATION OF A SOLUTION
Data Table
Name:
Date:
Period:____________
Part I: Data Table – Validating the Beer-Lambert Law
Absorbance as a function of [Co2+] measured at
Tube No.
[Co2+]
500
nm.
Absorbance
1
2
3
4
5
Use the data above to construct a graph of Concentration of CoCl2 vs.
Absorbance. Draw the best straight line that will go through the origin and as close as
possible to all other data points.
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Incorporating Engineering in High School Science and Math
Part II:
Determining the Concentration of an Unknown
Unknown #
Absorbance
A
B
C
Calculations:
1. Attach the graph from Part I.
2. Find the concentration of the unknown(s) by reading from the calibration curve as
explained in number 9 of the Procedure.
Questions:
1. In the graph for Part 1, why must the straight line pass through the origin?
2.
Based on the level of cobalt chloride you calculated in each of these Samples,
describe the efficiency of this water treatment method for removing this
contaminant.
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