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Determining the Concentration of a Solution: Beer’s Law
Determining the Concentration of a Solution: Beer’s Law
Learning Objectives:
Learning Objectives:
Upon completion of this experiment, students will:
(CLO3). Analyze evidence to decide if generalizations or conclusions based on the obtained data are warranted
(CLO4). Interpret and utilize mathematical formulas while solving problems
(MLO) Calculate the molarity of a solution using molar mass and the mole concept: convert between mass, moles, and molarity.
(MLO) Determine the concentration of a solution that has been diluted in addition to applying dilution principles toward serial dilutions.
Utilize graphing techniques, interpolation and extrapolation to determine the concentration of an unknown solution using Beer’s law.
Experiment 13 using a technique that is present throughout scientific measurement. A Beer's Law plot looks at the absorption or emission of light from a substance as a function of its concentration. Making solutions, dilutions and using a spectrophotometer are the tasks essential to this laboratory assignment.
The assignments that must be completed are:
The assignments that must be completed are:
The pre laboratory writing assignment must be completed before the laboratory session - This is the introduction for the formal report that must be written for this experiment.
Complete the pre laboratory problems
Complete and collect the necessary data to complete the tables and questions in the laboratory report. Create the Beer's law plot from the data and determine the concentration of all three unknown solutions.
Complete the formal report assignment for this experiment. Appendix D of the laboratory manual explains how to write a formal laboratory report.
You can use Activity 4 to help you create the graphs and charts for this experiment.
Discussion
Discussion
When atoms are heated or great amounts of electricity are passed through substances, the electrons within the atom can move inside the electronic structure of the atom. The energy of the electrons is said to be quantized and the electrons are only able to move in specific orbits or orbitals. For an electron to move away from the nucleus, the electron must absorb energy, while the electron emits light at various wavelengths when the electron moves toward the nucleus. Quantum theory explains this process. Every atom has a specific pattern of absorbed energy and emitted light. When the energy is absorbed, we can plot the relationship between intensity and wavelength as an absorption spectrum and we can plot the wavelength or frequency of the light emitted as the emission spectrum. These two spectra are related to each other as a picture and negative are related.
Electromagnetic radiation is a continuous spectrum of many wavelengths including visible light, ultraviolet, infrared, radio waves, x rays and gamma rays as a few examples. Each of these types of "light" have unique energies, wavelengths and frequencies which are all related. The energy of the radiation is proportional to the frequency, E = hν, where E is energy, ν is frequency measured in Hertz or cycles per second, and h is a constant, Planck's constant. The frequency is inversely proportional to the wavelength of the radiation, νλ = c, where ν (nu) is the frequency, λ (lambda) is the wavelength measured in meters, and c is the speed or velocity of light in a vacuum, 2.996 x 108 m/sec or 186,000 miles per second. Visible light has wavelengths between ~250 nm to ~ 800 nm. The absorption plot will graph the absorbance at different visible wavelengths, for example, this is the visible absorption spectrum of chlorophyll and other colored compounds.
When light passes through a substance, it can either be absorbed, reflected or transmitted. Experimental measurements are usually made in terms of transmittance (T), which is defined as:
%T = I / Io x 100
where I is the light intensity after it passes through the sample and Io is the initial light intensity. The relation between A and %T is:
A = 2 - log T = 2 - log (I / Io).
Absorption of light by a sample
Modern absorption instruments can usually display the data as either transmittance, %-transmittance, or absorbance.
The bonding structure in molecules have similar absorption spectra which can be used to determine structure of the molecule. There is an added relationship between the amount of the molecule present and the amount of light that is absorbed.
The above videos show how light interacts with substances and the equipment, tools and techniques we use to measure this interaction.
Beer's Law shows that the concentration is proportional to the absorbance or the amount of light that is absorbed, A = εbc, where ε is a constant called the absorptivity constant, b is the distance through the solution which light must pass through, the pathlength, and c is the concentration of the solution generally given in molarity. Beer's Law is a linear function where A is plotted on the y - axis and c is plotted on the x - axis where εb is the slope of the line. b is usually given the value of 1 cm because this is the diameter of the curvette used to measure the absorbance of the solution.
The way in which the absorptivity depends on wavelength defines the spectrum of the substance in question. Most substances show a maximum in ε over a sufficiently broad wavelength range. The wavelength at that maximum value is called the analytical wavelength of the substance and denoted λmax. Normally, Beer's law is applied at the analytical wavelength of a given material. The sensitivity to concentration differences should be largest at that wavelength.
Concentration (M) Absorbance
0 0
1.2 X 10-3 0.15
2.4 X 10-3 0.31
4.8 X 10-3 0.70
9.6 X 10-3 1.25
1.9 X 10-2 2.62
A standard solution is made of exact concentration. To create the Beer's Law plot, the original solution is diluted to various concentrations. Dilution occurs when a small amount of a concentrated solution has water added to increase the volume. If the dilution is done quantitatively using either a pipet and volumetric flask or a buret, then the number of moles in the concentrated solution will equal the number of moles in the dilute solution. If the number of moles is determined by the relationship
moles = Molarity x Volume, then in dilution
Molarity (concentrated sample) x Volume (concentrated Stock solution) = Molarity (diluted sample) x Volume (final diluted sample)
Absorption and Emission Spectra
https://youtu.be/1uPyq63aRvg
How to use a Spectrophotometer: https://youtu.be/xHQM4BbR040?list=PL967C03F996296203
Beer's Law Theory
Pre Laboratory Writing Assignment
Pre Laboratory Writing Assignment
The pre writing assignment must contain three parts: a purpose statement of what the objectives of the assignment are, a theory paragraph containing background and an understanding of why we are completing this assignment and finally, a summary paragraph of the procedure explaining how the assignment will be completed.
A pre writing assignment is included in the example of a formal laboratory report at https://docs.google.com/document/d/1BGBPJS3VqNME4cKDrR50HqqrIYZt0sZ9UCg6IrS3MDk
This assignment is the beginning of your formal report and must be included in the formal report. It will only be accepted before the laboratory session begins. No late assignments will be accepted.
Pre laboratory assignment
Pre laboratory assignment
The pre writing and the pre laboratory problems are questions about the laboratory assignment, background, definitions or procedure and calculations. These problems should be completed BEFORE coming to the class where the assignment will be discussed.
Before you come to lab to complete this experiment, there are several calculations that must be completed. We will use MicroLab to complete this experiment and the concentration of the solutions must be determined before their absorption can be measured. The experiment begins with you preparing your own stock solution of cobalt (II) nitrate using the hydrated salt.
Calculate the mass of Co(NO3)2 6H2O needed to make 50 mL of a 0.150 M stock solution using a 50 mL volumetric flask. After you have prepared the solution, determine the actual concentration of the stock solution from your measured mass.
Prepare the diluted solutions in test tubes labeled 0 - 5.
Test tube Volume of stock solution Volume of water
0 0 mL 5 mL Blank solution
1 2 mL 8 mL
2 4 mL 6 mL
3 6 mL 4 mL
4 8 mL 2 mL
5 5 mL 0 mL Stock solution
Calculate the concentration of each diluted solution using the actual concentration of your prepared solution.
Two graphs will be prepared from data collected using a spectrophotometer. The spectrometer directions will be given in lab, however, the general directions for the use of the MicroLab program is given in the laboratory manual or as a Google doc at https://drive.google.com/open?id=1dQaLDKJgltjoB_dtczQ9tDBsypP7cPVQO8buEzRSkLk
The first graph will be prepared from absorbance reading plotted for each wavelength of light used between 400 and 700 nm. This graph is used to determine the wavelength at which there is a maximum absorption of light.
The Beer's Law Plot is created by plotting Absorbance for each of the five test tubes prepared above against the concentration determined for each test tube. According to Beer's Law, A = 𝛆bc, as the concentration decreases, the absorbance will decrease proportionally. The slope of the "best fit" line, 𝛆b, is called the absorptivity.
An unknown solution absorbance is measured and the Beer's Law plot is used to determine the concentration of the unknown solution by extrapolation.
Formal Report and Conclusion Paragraph
Formal Report and Conclusion Paragraph
Complete the formal report and conclusion paragraph. A discussion of how to prepare the formal report and a conclusion paragraph is given in Appendix D: How to Write a Formal Laboratory Report
Links to website resources
Links to website resources
https://sites.google.com/a/email.vccs.edu/college-chemistry-ii/home/laboratory-reports
A sample Formal Report
A sample Formal Report
https://docs.google.com/document/d/1BGBPJS3VqNME4cKDrR50HqqrIYZt0sZ9UCg6IrS3MDk
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