PhET: Solution Concentrations and Beer’s Law in UV-VIS Spectrochemical Analysis Dr. Jursich UIC, Dept. of Chemistry Learning Objectives: Understand effects of dilution, draining, and solvent evaporation on solute moles and molarity concentration of solutions. Demonstrate principles of Beer’s Law and apply it to the analysis of different solutes in aqueous solutions. Experimental Objectives: Quantitatively measure mass of known volumes of two unknown liquids in order to distinguish between the two liquids. For a tutorial on introduction on Beer’s Law see: Labflow video on Beer’s Law and Standard Curve Analysis, Chem Libre textbook at https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supple mental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Electro nic_Spectroscopy_Basics/The_Beer-Lambert_Law Procedure: Go to https://phet.colorado.edu/en/simulation/beers-law-lab . There you will find the cover page as shown below. This will be a three part experiment. Part 1: Examine influence of dilution, evaporation, and draining on the concentration of solution Part 2: Demonstrating linearity of Beer’s Law Part 3: Determine concentration with Beer’s Law Click on “Concentration to start Part 1 and it brings you to set-up on right side. Here, several control features are highlighted in red and described below. “Faucet handle” allows addition of solvent “Drink mix” identifies solute to be added to solution. Shaking it adds solute to solution. “Solute Selector” (upper right). Allows one to select solute. Currently on “Drink mix” but can be change to numerous more interesting compounds by clicking on the downward triangle at right. “Concentration Sensor” Click and dragging this sensor into the solution will allow concentration to be read on meter. “Drain valve” allows one to drain out solution. “Evaporation” allows one to remove solvent from solution. And since all solutes are nonvolatile solids, all solute stays in the container during evaporation. Part 1: In this part of experiment you will prepare a solution with drink mix and perform dilution, solution removal by draining, and evaporation. Each time you will need to keep track and calculate moles of solute and molarity concentration in order to observe how they change from these changes of solution. Before starting you should prepare a table as shown below to report your results. 1a: Preparing a solution: First move the sensor into the solution. The concentration should read zero since no solute is added. Give the drink mix a few shakes and record the concentration you have for your solution in your notebook. Note the volume of the solution and from that calculate the moles of solute (drink mix) there are in the solution. 1b: Effect of dilution: Using the faucet add about 200 mL of water and record the sensor concentration. Estimate and record the final volume of solution. Record the concentration. Based on initial and final volumes calculate concentration and compare with the sensor reading. It should be very close but not necessarily exactly the same due to uncertainty in volume reading. That’s ok. Based on the new volume and sensor concentration calculate the moles of solute after dilution. 1c: Effect of draining solution: Using the drain valve remove about 300 mL of solution and record the new volume and sensor concentration. Note how the sensor concentration did not change. Calculate the moles of solute in the remaining solution after draining part of solution. 1d: Effect of evaporation: Using the evaporation panel slowly evaporate solvent to reduce volume of solution to around 250 mL. Record the sensor concentration and new solution volume. Calculate the moles of solute based on new volume and sensor concentration. Did it change significantly? Why or why not? Summarize the results of Part 1 by filling in Table below. Solution Step preparation dilution draining evaporation Initial Vol (mL) 500 (Final Vol. (mL) Sensor Reading Molarity of Solution (mol/L) Moles of solute Part 2: Demonstrating Beer’s Law Now we go back and to beginning cover page and select “Beer’s Law”. You should see the default settings as on the middle image below. For this part of lab, click on each highlighted item to the settings as on right side image. 1. Turns on the light source 2. Change detector reading to absorbance, A. Recall from introduction slides (A = -log10(transmittance)) 3. This is the sample description panel. Click on upward triangle and select Co(NO3)2. The lower portion of panel has a concentration selector. Currently, it is 100 mM by default. The concentration can be changed simply by clicking the left or right triangles or click and drag the slide bar. 4. Click and drag arrow to change pathlength. (You can click and drag rule to use it to measure pathlength in cm. 5. This is the wavelength selection panel. Change from preset to variable. Now one can vary the wavelength by clicking the left/right triangles or click and drag slide bar. 2 1 5 4 2a: Obtaining absorbance spectrum of 100 mM Co(NO3)2. To collect an absorbance spectrum of the Co(NO3)2 solution, a series of absorbance measurements need to be recorded over different wavelengths. Set the wavelength of light to 400 nm. Record the absorbance reading. Make a table of wavelength (nm) and Absorbance in your notebook. Then progressively record absorbance about every 50 nm from 400 to 750 nm. You’ll see absorbance increase then decrease with increasing wavelength. Near the peak of the absorbance, record absorbance every 10 nm to better define the maximum. In excel or other plotting program, plot the absorbance spectrum from 400 to 750 nm. Every compound has its own unique spectrum depending its properties of its electrons. For this solute the absorbance is coming from the Co2+(aq) ion. Identify the wavelength of maximum absorbance. 2b. Beer’s Law Relationships. Beer’s law states that A = εcl where A is absorbance, ε is extinction coefficient which is unique for each compound and dependent on wavelength, and l is the pathlength where light passes through the sample. First, set the wavelength of light to where maximum absorbance is observed. Then with same 3 concentration of Co(NO3)2 (100 mM) vary the pathlength to 0.50, 1.0, 1.5, 2.0 cm and make a table of pathlength and absorbance. For your report, make a plot of Absorbance (y-axis) versus pathlength (x-axis). Next, let’s verify the linear concentration dependence. Keeping pathlength at 1 cm and wavelength at maximum absorbance, vary the concentration of the solute from 50 to 300 mM every 50 mM. Make a table of wavelength (nm) and absorbance for your report. Make a plot of recorded Absorbance (y-axis) versus Concentration (x-axis) in mM and from the slope of your graph determine the extinction coefficient, ε, in units of M-1cm-1. Part 3: Determining Concentrations with Beer’s Law Given data you collected in Part 2, determine concentration of Co(NO3)2 analyzed in the following solutions. Sample 1 2 3 4 Pathlength (cm) 1.00 2.00 0.50 1.50 Absorbance measured at wavelength giving maximum absorbance 0.252 0.132 0.985 0.723 Concentration (mM) Lab Report Expectations: Report to be typed or neatly hand written/printed or combination of both. In the end, the entire report needs to be composed into a single pdf document. For your lab report, make sure to organize into the following components as a single file document such as Word. Then convert it into a single pdf file and upload into Labflow. Header Information: Your FULL name, Lab section (TA/day/time), FULL name of expt, date of expt Purpose: In one or two sentences, state in own words on the purpose of the experiment Procedure: For each Part of experiment, give a one sentence general description what was done. No need to give specifics as given in this procedure. Results: Organize your data into separate Parts 1, 2, and 3. Present the data in tabular and graphical forms as described above for each Part. Show example calculations where calculations are done. Be sure to show units in calculations. Conclusion: Summarize results in terms of the following points. How does concentration and moles of solute change for a solution undergoing dilution, draining, and evaporation? What is the pathlength and concentration dependence on light absorbance of a solution? How would one rearrange Beer’s Law equation to determine concentration of a light-absorbing solute?
Purchase answer to see full attachment