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17 Nov 2019

ortho and meta cresol have similar but not identical UV spectra. a 5.00 ml sample was dissolved in enough isooctane t make the final volume 25.00ml. Calculate the molar concentration of each isomer in the original sample of the unknown from the following data obtained from the diluted sample. a 1.00 cm cell was used. o-cresol: concentration(M): 4.75 x 10^-4; absorbance at wavelength 1: 0.796; absorbance at wavelength 2: 0.415 m-cresol: concentration (M: 5.88 x 10^-4; absorbance at wavelength 1: 0.375; absorbance at wavelength 2: 0.645 unknown: concentration: ?????; absorbance at wavelength 1: 0.727; absorbance at wavelength 2: 0.530

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Deanna HettingerLv2

1 May 2019

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1. The following question is about Beer's Law. What is Beer's Law? P.113 a. b. What is meant by deviations from Beer's Law? c. What are the three assumptions underlying Beer's Law? 3 in a lem cell. Calculate the molar absorptivity at 266nm. Ar & bc length and concentration were cut in half? ã« 2. The absorbance of a 2.31x10 M solution of a compound is 0.822 at a wavelength of 266nm 3ã The absorbance of a solution is 0.740, what would be the absorbance if both the path ã4. A 3.96x10-M solution of compound A exhibited an absorbance of 0.624 at 238nm in a Iem cell. A solution containing only solvent (i.e., blank) had an absorbance value of 0.02 at the same wavelength. Find the molar absorptivity of compound A. 5. Using the following table of prominent absorption wavelengths and molar absorptivities for naphthalene and anthracene, suggest a wavelength at which anthracene can be determined in the presence of an unknown amount of naphthalene and calculate the minimum detectable concentration of anthracene at the optimum wavelength. Assume a cell path length of 100 cm and the minimum detectable concentration corresponds to an absorbance of 0.0132. Absorption Wavelength Molar Absorptivity 221 286 312 375 221 312 375 Compound Naphthalene 9300 290 Anthracene 15,000 1200 9000 A compound with a molecular weight of 292.16 was dissolved in a 5ml volumetric flask. A Iml aliquot was withdrawn, placed in a 10ml volumetric flask and diluted to the mark. The absorbance at 340nma was 0.427 in a 1 cm cuvette Îµ for this compound at 340nn is 6130M'cm 6. Calculate the concentration of the compound in the cuvette What is the concentration of the compound in the 5ml flask a. b. c. How many milligrams of compound were used to make the 5ml solution? An acid, HTS, is added to 1M acetic acid/acetate buffer (pH 4) to yield a 0.001M solution. HTS does not absorb whereas TS does absorb in the visible region (A-540c for 1- 650nm with c is in molarity.) If the absorbance signal from the 10 3M solution is 0.27, what is K of HTS? For an 8x10- M methyl red solution at pH 5, As25am is 0.92. (1cm cells were used to make this measurement.) The experimenters ascertained the following two facts about 7.

If a solution absorbed violet light, what color would the solution appear to be? Hint: Table I, at the beginning of the experiment, will help you answer this one.

Calculate the concentration of a solution prepared by adding 15.00 mL of 2.10Ã10â32.10Ã10-3 M KMnO4KMnO4 from a buret into a 50.00 mL volumetric flask, which is then filled to the 50.00 mL graduation mark with distilled water. Hint: This is a dilution calculation.

________M

A linear standard curve of KMnO4KMnO4 is prepared from a set of standard solutions by plotting absorbance (y) vs. concentration (x). Use Google Sheets/Excel to plot this standard curve using the data in Table 1. Help with spreadsheets [PDF]

Table 1: Data for the standard curve of KMnO₄

Absorbance. Concentration (M) 0.318. 6.00 x 10^-4. 0.254. 4.80 x 10^-4. 0.191. 3.60 x 10^-4. 0.127. 2.40 x 10^-4. 0.0636. 1.2 x 10^-4

After plotting the standard curve, enter its slope below (not the equation of the line).

Slope = _______M^-1

Suppose that an unknown sample is analyzed spectrophotometrically at the same wavelength that was used for the standard curve and the absorbance was found to be 0.184. Use the standard curve to find the concentration of the unknown. Hint:Use y = mx + b to solve for the concentration (x) from the absorbance (y) and the m and b values obtained in question 2.

_____^M

THE ONE IN ITALICS AND BOLD IS THE PRELAB QUESTION PLEASE ANSWER THAT. PLEASE HELP ME ANSWER THE POST LAB QUESTIONS. I did THE PRE LAB. I ALSO FILLED in the tables.

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.)

SPECTRONIC is a registered trademark of Spectronic Instruments, Inc.

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 SPECTRONICÂ® 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 (Kimwipe^TM) 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 Switchcounterclockwise 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	88.3	0.055
425	73.7	0.132
450	43.6	0.362
475	23.8	0.623
500	15.7	0.804
525	16.0	0.797
550	38.9	0.410
575	73.0	0.37
600	85.3	0.069

MAXIMUM ABSORBANCE IS LOCATED AT __________500______nm.

TABLE 3 -Results measured at _500_____nm.

Test tube No.	% Transmittance	Absorbance	Molarity(M)
1	15.8	0.802
2	22.8	0.643
3	27.4	0.562
4	33.1	0.480
5	40.2	0.396
6	47.6	0.323
7	69.3	0.159

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 Â¾, or by Â½, and by Â¼ 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			51.9	0.284
2			70.7	0.151
3			79.3	0.101

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 _____B_____

TABLE 4

Diluted concentration (M)	Transmittance (%T)	Absorbance (A)
Pure unknown B	8.7	1.062
1 mL of unknown in 1 mL of water	29.5	0.531

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:

If needed I did a graph slope and got y= 0.1896x

R^2=0.9951

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 (M_c x V_c = M_d x V_d) where M_c is the molarity of the concentrated solution, V_c is the volume of the concentrated solution, M_d 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.

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