LAB 4:
SOLUTION CONCENTRATION AND BEER’S LAW
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This lab does NOT require a lab report.
You will work in groups of 3 or 4.
I Introduction
When you shine light through a solution, some of the light is absorbed. The amount of light
absorbed depends on three characteristics of the solution: the identity of the solute, the
distance through the solution the light must pass, and the concentration of the solution. The
relationship between these quantities is known as Beer’s Law and is represented by the
following equation:
A = εlc
where A is absorbance (a unitless quantity), ε is the molar absorptivity of the solute (a
constant unique for each solute and with units of 1/cm∙M), l is the pathlength through the
solution the light must travel (in units of cm), and c is the concentration of the solution (in
units of molarity).
The primary objective of this experiment is to determine the concentration of an unknown
nickel(II) sulfate solution using Beer’s Law. To gather the required data, you will use a
colorimeter like that shown in Figure 1. In this device, red light from an LED light source will
pass through the solution and strike a photocell. The NiSO4 solution used in this experiment
has a deep green color, meaning that it absorbs red wavelengths of light and allows other
colors (such as green) to pass through it. The colorimeter measures the amount of light
transmitted through the solution and received by the photocell as an absorbance value.
Figure 1
Figure 2
You will prepare five nickel sulfate solutions of known concentration (standard
solutions) and measure their absorbances in the colorimeter. You will then construct a
calibration graph of absorbance vs. concentration, as shown in Figure 2. You will then
determine the molar absorptivity, ε, of the solution from the graph’s slope using
Beer’s law.
Finally, you will determine the concentration of an unknown NiSO4 solution by
measuring its absorbance with the colorimeter and using the value of ε you
determined in the first part of the lab.
A secondary goal of this lab is to acquaint you with the data acquisition and analysis
tools of the Labpro Data Interface, Datamate (or Easydata) calculator program, and
colorimeters, all of which we will use in several future experiments.
Equipment and Reagents
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LabPro interface
TI Graphing Calculator
Vernier Colorimeter
one cuvette
five test tubes
30 mL of 0.40 M NiSO4
5 mL of NiSO4 unknown solution
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two 10-mL mohr pipets
two 100-mL beakers
pipet bulb
distilled water
test tube rack
stirring rod
! Warnings!
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Nickel compounds are toxic by ingestion and are skin irritants. Inform your teacher
immediately in the event of an accident. Wear your safety goggles. Aprons are
optional but always encouraged.
Procedure
Part 1: Constructing the Calibration Curve using the Standard Solutions
1. Prepare the standard solutions.
a. Add about 30 mL of 0.40 M NiSO4 stock solution to a 100-mL beaker. Add about 30
mL of distilled water to another 100-mL beaker. The exact volumes are not
important; you will only use these solutions to make your five known (standard)
solutions.
b. Label four clean, dry, test tubes 1-4 (the fifth solution is the leftover stock 0.40 M
NiSO4 in the beaker).
c. Pipet 2, 4, 6, and 8 mL of 0.40 M (stock) NiSO4 solution into Test Tubes 1-4,
respectively. With a second pipet, deliver 8, 6, 4, and 2 mL of distilled water into
Test Tubes 1-4, respectively. Volumes for each test tube (or each trial) are
summarized in the data table on the accompanying “Data and Calculations” page.
Note that all test tubes should contain 10 mL of solution when finished. Thoroughly
mix each solution with a stirring rod. Clean and dry the stirring rod between
stirrings.
d. Keep the remaining 0.40 M NiSO4 in the 100-mL beaker to use in the fifth trial.
e.
Calculate the concentrations of each diluted solution before coming to lab and fill
in the values in your data table.
2. Setup the calculator and colorimeter:
a. Plug the Colorimeter into Channel 1 of the LabPro interface.
b. Use the link cable to connect the TI Graphing Calculator to the interface.
Firmly press in the cable ends.
c. Turn on the calculator. Hit the APPS button to see if the DATAMATE or
EASYDATA program is installed on your calculator. If it is, start it by selecting
it, press CLEAR to reset the program, and skip to step 4. If DATAMATE is not
installed on your calculator, see Mr. Benter.
3. Calibrate the colorimeter using a blank (a cuvette of only distilled water):
a. Prepare a blank by filling an empty cuvette ¾ full with distilled water. To
correctly use a colorimeter cuvette, remember:
All cuvettes should be wiped clean and dry on the outside with a tissue.
 Handle cuvettes only by the top edge of the ribbed sides.
 All solutions should be free of bubbles.
 Always make sure the level of solution is between the black lines on the
side of the cuvette.
b. Place the blank in the cuvette slot of the Colorimeter and close the lid. Always
position the cuvette with its reference mark facing toward the white
reference mark at the right of the cuvette slot on the Colorimeter.
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c. Select SETUP from the main screen.
d. If the calculator displays COLORIMETER in CH 1, set the wavelength on the
Colorimeter to 635 nm (Red). Then calibrate by pressing the CAL button on the
Colorimeter until the red light on the colorimeter flashes. If the calculator
does not display COLORIMETER in CH1, let Mr. Benter know.
e. Your colorimeter is now calibrated. It is operating on the red LED and will
subtract the absorbance of distilled water from each of your following
solutions.
4. Prepare the next solution:
a. Empty the previous solution/water from the cuvette in your waste beaker.
b. Using Q-tips, thoroughly dry the insides of the cuvette.
c. Fill the cuvette 3/4 full with the next solution (it should be between the black
lines).
d. Wipe the outside of the cuvette with a tissue.
e. Place the cuvette in the Colorimeter, lining up the reference mark on the
cuvette with the reference mark in the colorimeter. Close the lid of the
colorimeter.
f. When the absorbance value displayed on the calculator screen has stabilized,
record the value in your data table.
5. Repeat Step 4 for each of your 5 standard solutions. Note: Wait until Part 2 to do
the unknown.
Part 2: Determining the Concentration of an Unknown
6. Determine the absorbance value of the unknown NiSO4 solution. To do this:
a. Obtain about 5 mL of the unknown NiSO4 in another clean, dry, test tube.
b. Prepare your cuvette as described in Step 4 above.
c. Monitor the absorbance value displayed on the calculator. When this value has
stabilized, record it in your data table (round to the nearest 0.001).
d. Dispose of the remaining solution in the waste beaker.
Part 3: Processing Your Data
7. Tomorrow, we will venture to the computer lab to construct a calibration graph of
absorbance vs. concentration. We will use this graph to calculate the value of є for
NiSO4 and then determine the concentration of the unknown solution. Note that
no lab report will be required 
Data and Calculations (to be recorded on your lab Data Sheet)
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Trial
Volume of Stock
NiSO4 Solution
Volume of
Distilled Water
1
2
3
4
5
Unknown
2 mL
4
6
8
From beaker
~5
8
6
4
2
0
0
Concentration
(mol/L)
Absorbance
Title and Purpose
Procedure
Above data
Graph of data
Slope of calibration curve
Molar absorptivity, ε, of NiSO4
Calculated concentration of unknown (show work)
? Questions to Answer
1. Many of you correctly noted in Lab 3 that adding extra water when dissolving
the copper chloride salt did not affect your results. Suppose a similar scenario
in this lab:
The test tubes you are using to make your standard solutions have a small
amount of water in them. The voice on the left side of your head tells you to
go ahead and use them: the water won’t make a difference since you’re adding
water anyway. The voice on the right side of your head, however, says that you
need DRY test tubes. Which of your voices should you listen to? Will the water
increase your calculated concentrations, decrease your calculated
concentrations, or have no effect on them? Explain.
2. When placing the Unknown solution into the cuvette, you touch the wrong side
of the cuvette and get fingerprints on the “transparent” side. To make matters
worse, you just finished off the bag of Cool Ranch Doritos that you snuck into
lab (yet another reason why NOT to eat in here).
As a result of your fingerprint, will your calculated concentration of the Unknown be
too high, too low, or unchanged? Explain.