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Beer’s Law: Determining the Concentration of a Solution
-The course and section number
-Your instructor’s name
The sun is the main source of light. Light is detected by our retina located at the back of eye.
Human retina can detect only a certain range of wavelengths. This range is called visible
spectrum of light as it can be seen with naked eye. A part of light is reflected from object and a
part is absorbed. This forms the basis for different colors1. Visible spectrum has a limited range
of colors. UV-visible spectrometry is the technique based on absorbance quality of light. It
measures how much light is absorbed by a certain chemical/substance. This helps in determining
the concentration of a specific substance in a solution. Calibration plot is used for this purpose. It
represents the data of spectrometer in a graphical presentation3. This method of measuring
concentration is based on a scientific law, called Beer’s law. This law states that amount of
energy absorbed and/or transmitted by a substance is proportional to the concentration of solute
in a solution or absorbance capacity of a solution2.
Where A= absorbance of light
ε= molar absorptivity
b= length of light path
Objective of this experiment is to learn how spectrometer works. This experiment also aims at
learning the practical application of absorbance property of light for the determination of
concentration of substance in a solution which is same as defined by Beer’s law. We will
determine concentration of unknown solution to meet these objectives.
2 EXPERIMENTAL METHOD
2. Experimental method:
To calculate concentration of unknown solution, we first need to prepare stock solutions.
Determine the amount of Allura Red needed for 500mL of 1.9×10^-4M Allura Red stock
Weigh the calculated amount of Allura Red. This does not need to weigh exactly as calculated.
Taking a little extra does not matter at all. Measure weight of paper before and after adding
Allura Red to determine estimated weight of Allura Red.
Put some Allura Red into the 500ml stock flask. Measure weight of paper again to calculate the
amount of Allura Red used.
Add water to the flask. An estimated filling of half of flask would be enough. Just do not go over
the mark. Swirl the flask to dissolve the contents. Then, fill the flask up to the mark.
Make standard solution from stock solution. Take 4 100mL volumetric flasks and label them as
1,2,3,4. Rinse them and make them dry. Dispense 5mL, 10mL, 15mL and 20mL solution into
these flasks respectively b using pipet pump. Fill half of flask, swirl it well, then fill up to the
Set up the calorimeter. Set the absorbance of the solvent to the zero by using distilled water as
Measure absorbance of standard solution using calorimeter and create a calibration plot. Record
values in data table.
Measure absorbance of unknown solution. Plot absorbance value in data table for calibration
plot. This will give concentration on corresponding x-axis.
Table 1: preparation of stock solution
Mass of Allura Red (g)
Stock solution concentration
Table 2: standard solution absorbance and concentrations
Graph 1 of table 1:
4 EXPERIMENTAL METHOD
y = 0.0011x + 0.0076
R² = 0.9949
Equation of the line= y = 0.0011x + 0.0076
R2 value (correlation coefficient)= R² = 0.9949
Table 3: solution absorbance and concentrations for given unknown solution
Absorbance of unknown sample
Calculated concentration of
Concentration calculated by Beer’s law:
Where A=0.7 and εb is constant. The value of constant for this experiment is 0.0011. by putting
Our linear equation for this data is y=mx+b
Where y is distance from y axis and x is distance from x-axis. m and b give values of
slope of graph. Here in our equation, y represents the absorbance which is calculated by
spectrometer and x represents the concentration of solution that shows this absorbance.
Our equation shows positive slope. Linear slopes suggest that values are consistent
throughout observations. Unknown sample absorbs a certain wavelength of light which is
recorded. Our sample absorbed light of 0.7nm wavelength. The concentration of sample
is measured by plotting absorbance value in graph. Corresponding value on x axis with
reference to straight slope indicate the concentration of unknown sample. The validity of
results lies in a number of factors. If cuvettes used in the experiment are not properly
dried or cleaned, it can disturb values accuracy. Also, mixing techniques are important.
Poor pipetting and incorrect light source can cause errors in measurements. In this
experiment, special focus on above mentioned aspects must be ensured to avoid
maximum possible errors. R2 value and linear slope indicate that method is reliable.
In this experiment the relationship between absorbance of light and concentration of
solution was measured. The results indicate that there is direct relationship between the
two variables. An unknown sample was used to calculate concentration by Beer’s law.
Absorbance of solution was calculated by spectrometer and concentration was calculated
using the linear equation of data of known sample solution. Absorbance of unknown
sample calculated by spectrometer is 0.7nm. Concentration of solution was found to be .
6 EXPERIMENTAL METHOD
These results are consistent with theoretical values calculated by Beer’s law. This proves
the accuracy of the experiment and also confirms Beer’s law.
Sliney, D. H. What Is Light? The Visible Spectrum and Beyond. Eye 2016, 30 (2), 222–229.
The Editors of Encyclopedia Britannica. Spectrometer | Scientific Instrument. Encyclopædia
Rafferty, J. Beer’s Law. Encyclopædia Britannica; 2018.
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