Beer’s Law Lab
Using the absorbance of light to measure
concentration of a solution
Calorimeter
Absorbance and Concentration
• This lab is based
on the principle that
the “darker” a
solution, the more
concentrated it is
•“darker” solution contains more solutes more solutes
absorb more light  less light will be transmitted through it
•Concentration of solution can be measured
QUANTITATIVELY by measuring how much light is
absorbed
Creating a calibration curve
• First, calibrate your
colorimeter using solutions
with KNOWN concentrations
(standard solutions)
• You will calculate the
concentrations of your
standard solutions
beforehand (using the
dilution equation) and record
their absorbances
Finding the concentration
of your unknown
• Once you have the
absorbances of your
standard solutions, you can
measure the absorbance of
your unknown
• Using the graph/equation of
the trendline, you can obtain
the concentration of your
unknown
Beer’s law graph
-Chart title goes hereCalori M. and R. Cuvettes
0.6
0.5
Absorbance
y = 1.2109x
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
Concentration (mol/L)
0.3
0.35
0.4
0.45
Example
• If your unknown is measured to have an
absorbance of 0.250, what is its concentration?
• Use the graph
• Use the equation:
y = 1.2109 x
0.250 = 1.2109 x
(0.250) / (1.2109) = x
0.206 mol/L = x
(linear relationship)
Expectations
•
You will be evaluated on the following:
1. A pre-lab quiz testing your knowledge
of the theory & procedure (only 8 MC)
2. Your dilution calculations for your
standard solutions
3. Your graph
4. The accuracy of your unknown
concentration