Concentration of a Solution (Beer’s Law)
012-10736 r1.04
Concentration of a Solution (Beer’s Law)
Introduction
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Concentration of a Solution (Beer’s Law)
Lab Challenge
The more concentrated (darker) a colored solution is the more light the solution is
absorbing. Given the mathematical relationship between the absorbance of light and
the concentration of a solute, determine the concentration of a copper(II) sulfate
solution.
Copper(II) sulfate solutions
Concentration of a Solution (Beer’s Law)
Background
• Light is a form of energy that can interact with chemical substances.
• By examining the wavelength of light (its color) that is absorbed by a chemical
substance, the chemicals identity and quantity can be determined.
Concentration of a Solution (Beer’s Law)
...Background
• How a chemical appears to our eyes depends on how
light interacts with the substance.
• Light can be transmitted (passed through), reflected
(scattered), or absorbed by a substance.
Light transmitted
Light reflected
Light absorbed
Light absorbed
Concentration of a Solution (Beer’s Law)
Self-Check
1. The green light in the diagram is being
____________.
a) reflected
b) absorbed
c) transmitted
d) transmogrified
Concentration of a Solution (Beer’s Law)
...Background
The color observed depends on:
• the chemical nature of the
substance.
Co(NO3)2
• the concentration of the
substance in solution.
Concentrated
NiCl2
Dilute
K2Cr2O7
K2CrO4
CuSO4 KMnO4
• the thickness of the sample the light
passes through.
20 mm path length
55 mm path length
Concentration of a Solution (Beer’s Law)
...Background
•
These variables are mathematically related to one another in Beer's law:
A=ε×b×c
A = measured absorbance of the light
ε = absorbance coefficient (chemical nature of the substance)
b = path distance (thickness of the sample the light passes through)
c = concentration of solute
• This equation is a direct relationship (as c increases, A increases) supporting the
observation that more concentrated solutions absorb more light and hence look
darker.
Concentration of a Solution (Beer’s Law)
Self-Check
Beer's Law
2. According to Beer's Law, when you decrease
concentration c for a solution, what should result?
a) Absorbance constant is canceled out
b) Distance stays the same
c)
Absorption increases
d) Distance increases
e) Absorption decreases
A=ε×b×c
Concentration of a Solution (Beer’s Law)
...Background
In this experiment, a colorimeter will be used to find the mathematical
relationship between the absorbance of orange light (λ=610 nm) and the
concentration of copper(II) sulfate.
Light Source
Concentration of a Solution (Beer’s Law)
...Background
• Since the colorimeter has a fixed path length b, and the experiment only uses
copper(II) sulfate (ε) Beer's law (A = ε × b × c ) can be simplified to:
A = constant × c
• The direct relationship between absorbance and
concentration is mathematically represented by a straight
line: Y = mX + b.
• The mathematical relationship (equation) will be
determined by graphing measured absorbencies at known
concentrations of copper(II) sulfate and fitting the data to a
line (calibration curve).
• The equation of the linear fit will be used to determine the
concentration of an "unknown" copper(II) sulfate solution.
Y = mX + b
Concentration of a Solution (Beer’s Law)
Safety
• Follow all common laboratory safety procedures.
• Wash your hands with soap and water after handling the solutions, glassware,
and equipment.
• Copper(II) sulfate is hazardous to the environment and should not be disposed of
down the drain. Make sure you follow your teacher’s instructions on how to
properly dispose of the copper(II) sulfate solutions.
BE SAFE
Always wash hands
to remove residue
before leaving
Concentration of a Solution (Beer’s Law)
Materials and Equipment
Collect all of these materials before beginning the lab.
• Colorimeter
• Sensor extension cable
• Glass cuvettes with caps (7)
• Test tubes (6), 20-mm x 150-mm
• Test tube rack
• Beakers (2), 100-mL
Concentration of a Solution (Beer’s Law)
Materials and Equipment
Also collect these additional materials before beginning.
• Volumetric pipettes (2), 10-mL
• Pipette bulb or pump
• Non-abrasive cleaning tissue
• 0.80 M copper(II) sulfate solution, 30 mL
• Unknown copper(II) sulfate solution, 10 mL
• Distilled (deionized) water, 30 mL
Distilled
Water
Concentration of a Solution (Beer’s Law)
Sequencing Challenge
A. Determine the
linear equation for
the calibration
curve's line of best
fit.
B. Create a
calibration curve
by graphing
absorbance versus
concentration.
C. Measure the
absorbance of the
unknown and
determine its
concentration using
the equation.
D. Create five
copper(II) sulfate
solutions of known
concentration and
measure the
absorbance of
each.
The steps to the left are part
of the procedure for this lab
activity. They are not in the
right order. Determine the
correct sequence of the
steps, then take a snapshot
of this page.
Concentration of a Solution (Beer’s Law)
Setup: Calibration Curve
1. Connect the colorimeter to the data collection system using a sensor
extension cable.
2. Calibrate the colorimeter so that the colorimeter reads an absorbance of 0.0 for
distilled water (the solvent).
a. Fill a cuvette with distilled water and cap it.
b. Holding the cuvette by the lid, wipe the outside of
the cuvette with a non-abrasive cleaning tissue.
c. Place the cuvette in the colorimeter and close the lid.
d. Push the green calibrate button on the colorimeter.
e. When the green light turns off, the calibration is
complete and the cuvette can be removed from the
colorimeter.
Calibrate Button
Concentration of a Solution (Beer’s Law)
Setup: Calibration Curve
Q1: Why is it necessary to calibrate the
colorimeter using distilled water?
Concentration of a Solution (Beer’s Law)
Setup: Calibration Curve
3. Measure approximately 30 mL of 0.80 M copper(II)
sulfate stock solution into a 100-mL beaker.
4. Measure approximately 30 mL of distilled water
into a different 100-mL beaker.
5. Take six clean, dry test tubes and place them in a
test tube rack. Label the test tubes: 1, 2, 3, 4,
stock, and unknown.
Q2: Why do the test tubes
need to be dry? What
error would be caused
by wet test tubes?
Concentration of a Solution (Beer’s Law)
Setup: Calibration Curve
6. Prepare the five standard copper(II) sulfate (CuSO4) solutions listed in the
table below. Use one volumetric pipette for the CuSO4 and a different
volumetric pipette for the water (H2O).
Test Tube #
0.80 M CuSO4
H 2O
Concentration (M)
1
2.0 mL
8.0 mL
0.16
2
4.0 mL
6.0 mL
0.32
3
6.0 mL
4.0 mL
0.48
4
8.0 mL
2.0 mL
0.64
stock
10.0 mL
0 mL
0.80
7. Thoroughly mix each solution by gently swirling each test tube.
Concentration of a Solution (Beer’s Law)
Setup: Calibration Curve
8. Fill one cuvette with the 0.16 M CuSO4 solution and
cap it. Label the top of the cuvette lid.
9. Continue to fill one cuvette at a time until each
solution is in a cuvette, the cap is on, and it is
clearly labeled (0.32 M, 0.48 M, 0.64 M, & 0.80 M).
Q3: What will the
colorimeter measure?
Is this the
independent or
dependent variable?
Concentration of a Solution (Beer’s Law)
Collect Data: Calibration Curve
1. Use a non-abrasive cleaning tissue to wipe the
outside of the cuvette containing 0.16 M CuSO4
and place the clean cuvette inside the
colorimeter.
2. Close the lid of the colorimeter.
Q4: Why is it necessary to wipe the outside of the
cuvette before you place it in the colorimeter?
Q5: Why is it necessary to close the lid of the
colorimeter before recording the absorbance?
Concentration of a Solution (Beer’s Law)
3. Tap
to start a data
set.
4. Enter the molarity of the
first sample in the
appropriately labeled
column.*
5. Tap
to record the
absorbance of this solution.
*To Enter Data into a Table:
1. Tap
to open the tool
palette.
2. Tap
then tap a cell in the
data table to highlight it in
yellow.
3. Tap
to open the
Keyboard screen.
Concentration of a Solution (Beer’s Law)
6. Remove the 0.16 M
cuvette.
7. Wipe the cuvette of the
next sample and place
it in the colorimeter.
8. Enter the molarity of
this sample into the table.
9. Tap
to record the
absorbance of this
solution.
10. Repeat this process for
the last three solutions.
11. Tap
to stop the
collecting of data in this
set.
Concentration of a Solution (Beer’s Law)
Data Analysis:
1. Create a linear best fit line
of the data collected*.
Note: the equation of the best
fit line gives:
A = constant × c
y= m×x
*To Apply a Curve Fit:
1. Tap
to open the tool
palette.
2. Tap
to open the Curve
Fit screen.
3. Tap the name of the curve
fit required.
Concentration of a Solution (Beer’s Law)
2. What is the equation
for the line of best fit?
Replace y with
absorbance (orange
610) and x with
concentration.
3. Solve the equation
for concentration.
4. Enter the equation you
determined in question #3 into
the data collection system
calculator.*
Note: SPARK will use this equation to calculate
the concentration of your unknown.
*To create a Calculation:
1. Tap
to open the
Experiment Tools screen.
2. Tap CALCULATED DATA to
open the calculator.
3. Enter the calculation in the
space provided.
4. Tap Measurements to insert
collected data into the
calculation.
Concentration of a Solution (Beer’s Law)
Setup: Determining Concentration
1. Obtain 10 mL of the unknown copper (II) sulfate solution from your
instructor. Put this solution in the test tube labeled "Unknown Concentration".
2. Fill a dry, clean cuvette with the unknown solution, cap it, and label it
"unknown".
3. Wipe the outside of the cuvette with a non-abrasive cleaning tissue and place
the cuvette in the colorimeter.
4. Close the lid of the colorimeter.
Concentration of a Solution (Beer’s Law)
Collect Data:
*To Change the variable on a
digits display:
1. Change one of the digits
to open the tools
displays below to show the 1. Tap
calculated concentration of palette.
2. Tap
to open the Digits
your unknown solution.*
Properties screen.
2. Tap
to start a data set. 3. Tap the Measurement box and
select the desired variable.
3. Tap
set.
to stop the data
4. Enter the absorbance and
the concentration of the
unknown solution in the
text box on the top right.
Concentration of a Solution (Beer’s Law)
Analysis
1. State Beer's law. Does your data support this statement?
Concentration of a Solution (Beer’s Law)
Analysis
2. Would an error occur if some of the
cuvettes were dirty? Explain.
Concentration of a Solution (Beer’s Law)
Analysis
3. Explain the difference between “absorbance” and
“transmittance” of light.
Concentration of a Solution (Beer’s Law)
Analysis
4. Why is CuSO4 a bluish color? Are colors of light
other than blue transmitted?
Concentration of a Solution (Beer’s Law)
Analysis
5. Why was the absorbance of orange
(610 nm) light used instead of
another color?
Concentration of a Solution (Beer’s Law)
Synthesis
1a. A solution of sodium sulfate is clear and colorless, and yet a solution of copper(II)
sulfate is blue. Which ions are causing the blue color?
1b. Could you use Beer's law to find the concentration of a sodium sulfate solution?
Concentration of a Solution (Beer’s Law)
Synthesis
2a. What is the minimum number of points needed to create a calibration curve?
2b. How many points were used in this experiment? Why?
Concentration of a Solution (Beer’s Law)
Synthesis
3. Other sensors, such as for pH and conductivity, need to be calibrated, but are
able to determine unknown concentrations without creating a calibration curve.
Explain how this works.
Concentration of a Solution (Beer’s Law)
Multiple Choice
1. Which of the following variables affects the
absorbance of light in a solution?
a) The distance the light has to travel through
the solution (path length.
b) The amount of solute in each volume
(concentration).
c) The wavelength of the light that is interacting
with the solution.
d) All of the above variables affect the
absorbance of light in a solution.
Concentration of a Solution (Beer’s Law)
Multiple Choice
2. A sample 0.10 M nickel (II) chloride is placed into
a cuvette with a 1.00 cm path length. The solution
has a measured absorbance of 2.0. What would
you expect the absorbance of a 0.05 M nickel (II)
chloride solution to be?
a) 1.0
b) 2.0
c) 4.0
d) not enough information
NiCl2 solution
Concentration of a Solution (Beer’s Law)
Multiple Choice
3. What should be in the ‘blank’ cuvette when the
colorimeter is calibrated?
a) nothing
b) the solution with the greatest concentration
of solute
c) the solvent
d) a 1.0 M sample of the solution
??
Concentration of a Solution (Beer’s Law)
Multiple Choice
4. What color of light is transmitted through a
copper(II) sulfate solution?
a) red
b) orange
c) green
d) blue
Concentration of a Solution (Beer’s Law)
Multiple Choice
5. How many colors make up white light?
a) 1
b) 3
c) 4
d) more than five
e) none
Concentration of a Solution (Beer’s Law)
Congratulations!
You have completed the lab.
Please remember to follow your teacher's instructions for cleaning-up and submitting
your lab.
Concentration of a Solution (Beer’s Law)
References
All images were taken from PASCO documentation, public domain clip art, or Wikimedia Foundation Commons.
1. LIGHT BULB http://www.freeclipartnow.com/household/lightning/Lightbulb3.jpg.html
2. BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.htm
3. COLORED LIQUIDS http://commons.wikimedia.org/wiki/File:Coloured-transition-metal-solutions.jpg
4. VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html
5. COLOR SPECTRUM http://commons.wikimedia.org/wiki/File:EM_Spectrum3-new.jpg
6. EYE http://freeclipartnow.com/people/body-parts/eyes/big-blue-eye.jpg.html10.
7. BEAKER http://www.freeclipartnow.com/science/flasks-tubes/beaker-2.jpg.html
8. BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html