CHEM 2440 Midterm: CHEM 2440 Test 2 2003 Fall
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Gather 2 pieces of square filter paper, the scissors, a pencil, and a ruler. DO NOT use a pen
or any form of ink. Create a strip of filter paper that measures 14 cm by 7 cm. See Figure 3.
Use the ruler to measure 14 cm across the edge of the paper. Use the pencil to draw a small
âxâ at 14 cm. On an adjacent edge of the paper, use the ruler to measure 7 cm up the edge
of the paper. Use the pencil to draw a small âxâ at 7 cm. Using the ruler as a straightedge,
lightly trace a rectangle that is approximately 14 cm by 7 cm. Use the scissors to cut out the
rectangle. Create two additional strips of paper.
Note:There should be enough filter paper to create a fourth 14 x 7 cm strip of paper if needed.
2.Gather the 25 mL graduated cylinder, 100 mL beaker, distilled water, and salt. Use the
graduated cylinder to add a total of 50 mL of distilled water to the beaker. Add a pinch of salt
to the beaker. Use the stirring rod to stir the solution until the salt has completely dissolved,
resulting in a salt solution of approximately 0.1% NaCl.
3.Gather 2 sheets of the filter paper â 14 x 7 cm from step 1.
a.Create a base line by drawing a thin horizontal line with a pencil (NOT a pen!)
across the paper, 10 mm (1 cm) from the bottom. The line should barely be visible.
b.With a pencil, draw 9 small cross lines along the horizontal line, 1.5 cm apart,
Experiment
Chromatography of Food Dyes
4.Repeat step 3 for the second piece of filter paper.
5.On Sheet 1, from left to right, use a pencil to lightly label each section between the cross lines
with the abbreviation for the six FD& C food colors from your experiment bag that will be
tested: B1, B2, R3, R40, Y5, and Y6.
6.Also on Sheet 1, after Y6, place the labels KG and KS to represent the Kool-
Grape and Kool-Aid
®Strawberry drink mixes to be tested.
7.On Sheet 2 from left to right, use a pencil to lightly label each section between the cross lines
with the abbreviation for the grocery store food coloring that will be tested: SR, SY, SG, and SB.
8.Also on Sheet 2, after SB, place the label âUKâ for unknown and then âMâ plus a letter for the
color of 4 different candies from your bag of M&Ms: such as; M-R (for M&M® red), M-Y, M-B,M-G.
9.Set Sheet 2 aside and perform the following for Sheet 1. After you have completed the
experiment for Sheet 1, repeat for Sheet 2.
10.Put on your safety gloves and goggles.
11.For each dye on the sheet to be tested:
a.Use the scissors to cut open the pipets and place them in the 24-well plate, bulb
side down, using the well plate as a pipet holder.
b.Place a few drops of the dye in a well of the 24-well plate, as shown in Figure 4.
Dye drops and pipets in well
12.Once the paper is spotted with all the dyes, allow the spots to dry for a few minutes.
13.Form the spotted filter paper into a cylinder (colored spots on the inside) with the edges
touching, but NOT overlapped, and staple at the top and bottom as shown in Figure 7. (You
may want to use small pieces of tape on the outside to lightly hold the cylinder together
while you securely staple it. If so, remove the tape after stapling.) Set the cylinder aside for a
moment.
14.The salt water (~0.1% NaCl) you previously prepared will be your eluting solvent and a Petri
dish will be your elution chamber. Pour salt water into the clean and empty Petri dish to a
depth of about 1/4 cm. This is just enough to cover the bottom of the elution chamber.
15.Set your paper cylinder next to (not in) the Petri dish, with the spots at the bottom, and look
to make sure the solvent level is below the base line. If it is not, pour out a little solvent until
it is.The base line (and spots at the bottom of the chamber) must be above the solvent level
for this experiment to work.
16.Carefully place the paper cylinder into the eluting chamber, making sure not to touch the Petri
dish sides. See Figure 8. The solvent-front will travel up the paper rapidly at first and then will
slow down. Let the solvent-front rise, monitoring every few minutes. Immediately remove the
cylinder if any dye or the solvent front moves up higher than 2 cm from the top of the paper.
If all the solvent is soaked up before the front has time to move toward the top of the paper
carefully add a little more solvent to the Petri dish.
17.When the solvent-front has traveled up the filter paper, 2 cm from the top, remove the filter
paper cylinder from the chamber and immediately mark the top the solvent-front, on the dry
part of the paper, with a pencil. See Figure 9.
18.Allow the paper to dry, carefully remove the staples,
and draw an outline around each spot
using a pencil.
19.Take a photograph of your chromatogram after you have circled the spots.
Resize and insert the photograph of your chromatogram in Data Table 1of your
Lab Report Assistant
20.Use the ruler to measure the distance of the solvent front, in millimeters. Use Figure 1 in the
Background section as needed. Record the distance of the solvent front in Data Table 2 of your
Lab Report Assistant
For each of the dye spots on the filter paper chromatogram, measure and record to the
nearest millimeter the distance each of the spots has traveled up the solvent front. Start from
the original horizontal pencil line (base line) and measure to the top center of where the dye
stops in each column on the paper. See Figure 2 in the Background section as needed.
22.Calculate the Rf value for each spot and record in Data Table 2
23.Repeat steps 10 through 22 for the second sheet of filter paper. Record all data for this
chromatogram in Data Table 3 of your Lab Report Assistant
24. Compare the Rf values for the spots of the Kool-Aid ®, unknown sample, store food colorings,and M&Ms ®
, to those of the known FD&C food dyes, to determine which FD&C dyes are used in the Kool-Aid®, the grocery store food colorings, and in the M&Ms ®.
25.Record the FD&C dyes present in each of the samples in Data Table 4 of your Lab ReportAssistant
26.Clean up all equipment and return to the lab kit for future use.
27.When you are finished uploading photos and data into your Lab Report Assistant, save your
file correctly and zip the file you can send it to your instructor as a smaller file.
Enter the results from chromatography filter paper Sheet 1 of your experiment in the following tables. If more than one colour is present in a sample, please indicate colour when referring to distance and Rf measurement.
Sheet 1: FD&C Food Colours
| Colour | Blue | Blue | Red | Red | Yellow | Yellow | Solvent |
| Distance (mm) | |||||||
| Rf | -- |
Sheet 1: Drink Mixes
| Substance | Kool-Aid® | Kool-Aid® Strawberry | Solvent |
| Distance (mm) | |||
| Distance (mm) | -- | ||
| Rf | -- | ||
| Rf | -- |
Enter the results from chromatography filter paper Sheet 2 of your experiment in the following tables. Indicate the colours of M&Ms used in the experiment.
Sheet 2: Store Food Colours
| Substance | Store | Store | Store | Store | Solvent |
| Distance (mm) | |||||
| Distance (mm) | -- | ||||
| Rf | -- | ||||
| Rf | -- |
Sheet 2: Candy Colours and Unknown
| Substance | M&M | M&M | M&M | Unknown | Solvent |
| Colour: | Colour: | Colour: | |||
| Distance (mm) | |||||
| Distance (mm) | -- | ||||
| Distance (mm) | -- | ||||
| Rf | -- | ||||
| Rf | -- | ||||
| Rf | -- |
Answer the following questions based on your experimental results.
Show the calculation for the Rf value for Red40.
What FD&C colour(s) make up the unknown sample?
What FD&C colours make up the food colourings? State the brand used.
What FD&C colours make up the Kool-Aid® drinks and M&Ms®?
THIS IS A LAB REPORT THAT I NEED HELP WITH I NEED HELP WITH ANSWERING THE POST LAB QUESITON THAT IS ALL THE WAY IN THE BOTTOM PLEASE HELP ME ANSWER THE POST LAB QUESTIONS THANK YOU! THE PRE LAB I HAVE ALREADY DONE BTW!
Determination of Cobalt (II) Chloride
by UV/VIS Spectroscopy
Introduction
The absorption of specific quantities of electromagnetic (light) radiation by an element or compound allows electrons to move from lower energy level to a higher energy level. We say that the energy levels are quantized. Electrons, returning to lower energy levels, will release energy within the electromagnetic spectrum. The wavelengths may be in the ultraviolet, visible, or infra red regions of the spectrum. For cobalt (II) chloride, the compound of study, the visible region, of the spectrum will be addressed.
By knowing the wavelength (l) or frequency (n) of radiation, one may determine the energy of the transition. Relationship between energy, wavelength and frequency are:
c= ln and E= hn or E= hc/l
The light radiation absorbed in a solution of this salt is proportional to its concentration through a relationship provided by the Beer-Lambert Law:
A = kC
where A= absorbance, k is Beerâs Law constant, and C is the molar concentration in solution.
The absorbance (A), a unit-less number is dependant on the quantity of light energy absorbed and transmitted by the solution through the following equation:
A= -log T or A = 2-log %T
where T= transmittance= I/I0 where I is the absorbed light and I0 is the impinging light source.
Purpose
The purpose of this laboratory experiment is to practice, learn and carefully prepare molar solutions to investigate the relationship of concentration and spectrophotometer response. A 0.150 M solution of cobalt (II) chloride will be provided. You will prepare diluted solutions through the serial dilution method, measure the transmittance and calculate the absorbance values. You will prepare a Beerâs law graph of data; molar concentration vs. absorbance. Then determine Beerâs law constant using Microsoft Excel program.
You will also be given a concentrated sample of cobalt (II) chloride of unknown concentration and you must determine the appropriate dilution to prepare and thereafter determine the unknown concentration.
Procedure
In order to carry out this analysis procedure, it will be necessary to determine the wavelength of transition. Recall, that one specific energy maximum exists for each transition. We will study the transition in the range of 400- 600 nanometers (nm). This is the visible region of the spectrum.
Step 1. Prepare the following solution concentrations using the 0.150 M cobalt (II) chloride stock solution, 18 mm x 150 mm test tubes and Mohr pipets.
TABLE 1
| Solution No. | 0.150 M CoCl2 (mL) | Volume water (mL) |
| 1 | 5.0 | 0.0 |
| 2 | 4.0 | 1.0 |
| 3 | 3.5 | 1.5 |
| 4 | 3.0 | 2.0 |
| 5 | 2.5 | 2.5 |
| 6 | 2.0 | 3.0 |
| 7 | 1.0 | 4.0 |
Step 2. Carefully place a cork or rubber stopper over each solution and mix the contents.
Step 3. Measure the transmittance of a 0.150M solution between 400 nm and 600 nm at 25 nm intervals.
To accomplish this, follow these steps:
Caution: instruments are delicate. If you are uncertain about instrument operation, ask your instructor for assistance. (Instructions on instrument operation- see next page.)
|
Instrument operation- edited version of product *Operatorâs Manual
Transmittance and Absorbance
1. Turn on the instrument by turning the Power Switch (10) clockwise. Allow the spectro-photometer to warm up for at least 15 minutes to stabilize.
2. After the warm-up period, set the desired wavelength with the Wavelength Control Knob.
3. Set the filter lever to the appropriate position for the selected wavelength (not required for SPECTRONIC® 20D).
4. Adjust the display to 0%T with the Zero Control (10). Make sure that the sample compartment is empty and the cover is closed.
5. Set the display mode to TRANSMITTANCE by pressing the MODE control key until the appropriate LED is lit.
6. Fill a clean cell with water and wipe the cell with a tissue (KimwipeTM) to remove liquid droplets, dust and fingerprints.
7. Place the cell in the sample compartment and align the guide mark on the cell with the guide mark at the front of the sample compartment. Press the cell firmly into the sample compartment and close the lid.
8. Carefully adjust the display to 100%T with the Transmittance/Absorbance Control (9). Move the knob slowly as you approach 100%T.
9. Remove the cell from the sample compartment and empty the water.
10. Rinse the cell twice with small volumes of the solution to be measured and fill it with the solution.
11. Wipe the cell with a tissue and insert the cell into the sample compartment. Align the guide marks and close the lid.
12. Read the appropriate value (%T) from the display.
13. Remove the cell from the sample compartment and repeat steps 10 through 12 for any remaining sample solutions.
14. When all measurements are completed, turn off the spectrophotometer by turning the Power Switch counterclockwise until it clicks.
Step 4. Using solution no. 1, follow the directions for instrument operation and record (%) transmittance values for each wavelength in Table 2.
Remember to reset the instrument to 100% for each wavelength investigated.
DO NOT DISCARD THIS SOLUTION,
Step 5. The wavelength that gives the largest absorbance value will provide the greatest sensitivity to concentration change. Generate an Excel graph of wavelength (x) vs. absorbance (y) using these values. At zero absorbance, there isnât any CoCl2 so donât forget to include the (0,0) data point. Connect your data points with a smooth curve. This is the absorption spectrum of aqueous CoCl2. Submit this with your lab report.
Step 6. Reset your spectrometer using this wavelength (step 5), and measure this solution and the other six solutions listed in Table 1, and record your % transmittance values in Table 3.
TABLE 2
| Wavelength (nm) | % Transmittance | Absorbance |
| 400 | ||
| 425 | ||
| 450 | ||
| 475 | ||
| 500 | ||
| 525 | ||
| 550 | ||
| 575 | ||
| 600 |
MAXIMUM ABSORBANCE IS LOCATED AT ________________nm.
TABLE 3 -Results measured at ______nm.
| Test tube No. | % Transmittance | Absorbance | Molarity(M) |
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 | |||
| 6 | |||
| 7 |
Step 7. Plot a graph of Molarity (M) vs. Absorbance for all seven solutions using Excel plotting techniques. This is called a calibration curve, whereby one establishes a measured response to a known solution concentration. Determine the slope of the line and include the linear equation of fit, and the correlation coefficient.
Recall the relationship A = kC or y = mx + b; where b is the (0,0) data point
Then: k = A/C
Step 8. The unknown solution to be analyzed is more concentrated than any of the standard solution concentrations that were measured to prepare the calibration curve.
Points to consider:
You must achieve a dilution such that the absorbance is within the absorbance range of the standards that were previously measured.
By using serial dilutions, will multiply many errors; thus it is best to obtain a dilution is one-step if possible.
Consider reducing the concentration by ¾, or by ½, and by ¼ or to 0.10 of the original concentration and test the solution to see if the absorbance response is within range. You may also consider using the dilutions in Table 1. Once this is determined, proceed to the next step. Complete the dilution table for the diluted concentrations you prepared:
| Trial | mL of unknown | mL of water | (%T) | A |
| 1 | ||||
| 2 | ||||
| 3 |
Step 9. Using the desired dilutions, determined in step 8, measure two of the diluted solutions and record the measured transmittances in Table 4.
UNKNOWN NUMBER __________
TABLE 4
| Diluted concentration (M) | Transmittance (%T) | Absorbance (A) |
Calculation of absorbance, for diluted Unknown solution:
Step 10. Determine the concentration of the diluted solution using the linear equation of fit, knowing the absorbance and k, the slope of the line.
C = A/ k
Step 11: Determine the concentration in your original Unknown solution. Show your
back-calculation below to support your final answer to this experiment.
Calculations:
Unknown Solution No. ________ Concentration: ___________M
Prelab Questions
1. Consider a solution of 0.400M Co(NO3)2 provided by your lab instructor.
You are required to make the following dilute concentrations:
0.160M and 0.240M solutions. You are provided the following glassware: 5.0 mL Mohr pipet and 18 mm x 150 mm test tubes. Describe how you would prepare each solution, and support your description with your calculations. (Recall (Mc x Vc = Md x Vd) where Mc is the molarity of the concentrated solution, Vc is the volume of the concentrated solution, Md is the molarity of the dilute solution, and Vd is the volume of the diluted solution.)
Given 0.400 M Co(NO3)2 solution.
Lets say we pipette out 5 mL of this solution and dilute it to get 0.160 M solution. Then
Using, Mc x Vc = Md x Vd
0.400 x 5 = 0.160 x Vd or Vd= (0.400 x 5)/0.160 = 12.5 mL
Thus we need to add 7.5mL distilled water to 5 mL of cobalt nitrate. Since we are only provided with a 5 mL pipette, we can only measure out volumes in multiple of 5.
thus pipette out (5 x 2) 10 mL of cobalt nitrate solution in the test tube and dilute it by addition of (7.5 x2) 15 mL of distilled water.
Again for preparing 0.240 M solution
Lets say we pipette out 5 mL of this solution and dilute it to get 0.160 M solution. Then
Using, Mc x Vc = Md x Vd
0.400 x 5 = 0.240 x Vd or Vd= (0.400 x 5)/0.240 = 8.33 mL
Thus we need to add 3.33mL distilled water to 5 mL of cobalt nitrate. Again taking volumes only in multiples of 5, pipette out (5 x 3) 15 mL of cobalt nitrate solution in the test tube and dilute it by addition of (3.33 x3) 10 mL of distilled water.
2. In your own words, explain, in detail, what you will be learning during this lab. Address the chemical principles and, laboratory skills, in your answer.
Post - Lab Questions:
a) Letâs say, for a given concentration of salt solution, the maximum response at a wavelength maximum of 485nm, is 0.750. If an unknown solution is analyzed at a longer or shorter wavelength would the investigator obtain the same concentration for the Unknown solution? Explain why.
b) List all possible sources of errors that may result in this experiment.