Beers Law Assignment PDF

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Assignment on beer law experiment...

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Na me :Ani kaAn j um

Pa r t ne r :

St ude ntNo:20175044

St ude ntNo:

La bSe c t i on:19

Be nc h#( onc omput e rs c r e e n) : 38

Experiment 4: Beer’s Law. Purpose The purpose is to gain knowledge about the application of spectroscopic methods in determining the unknown concentration of a solute in solution by working with a simple spectrometer and using it to examine a solution chemically.

Introduction1 A set of four standard permanganate solutions having various known concentrations will be in this experiment. Firstly, a single absorption spectrum will be produced using each of the four permanganate solutions and then the max will be found out. An absorption spectrum is a graph that portrays the absorption of light penetrating a substance due to the substance absorbing certain wavelengths. On the other hand, max is the wavelength at which the absorption of light is the maximum which is a significant value since the Beer’s Law measurements taken at the max will have the minimum error for concentration and produce the best calibration curve. Next, a permanganate solution of unknown concentration will be made after which its absorption will be measured in order to find its concentration with the help of this calibration graph. Moreover, it is necessary to know the relationship of the Beer-Lambert Law, A x=xbCx (Equation 24), where Ax is the absorption,  is the extinction coefficient, b is the length of the medium, and Cx is the concentration of x in solution since this will assist in calculating the slope of the graph which in turn gives a solution for the concentration. Lastly, the equation of a line, y=mx+b is also required where (x, y) is a point on a graph, m is slope and b is the y intercept.

Procedure2 In Part A, a hot water bath is made and stirred using a stir bar. Next, a 100mL volumetric flask is filled with about 35mL of unknown Mn 2+ solution with the help of a 50 mL burette. 5mL of concentrated phosphoric acid is added directly into the flask from the dispensette after which the flask is swirled for mixing the contents. Then, 0.5g of KIO 4 is measured to be directly added into the flask. The flask is next placed in the hot water bath and heated for around 10 minutes, while swirling it from time to time. After 10 minutes, the flask is taken out and allowed to cool on the bench for further 10 minutes and RO water is poured up to the mark. The top of the flask is closed using a stopper in order to mildly shake and invert the flask for a couple of times while removing the stopper after each inversion to release pressure. In Part B, 4 cuvettes are washed with and every one of them is filled ¾ of the way full with the standard KMnO4 solutions from the 50mL beakers. A cuvette containing distilled water is put in the SpectroVis for

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calibrating the SpectroVis using Logger Pro. Then, the cuvette with the most concentrated KMnO 4 solution is placed in the SpectroVis and the data is recorded after a few seconds which created the graph showing maximum absorption. In Part C, the “most concentrated” cuvette is left in the SpectroVis and the Beer’s Law mode is set by setting the spectrometer for data collection of absorbance and concentration. Next, the absorbance is collected after it became stable and then the data for the other three known concentrations are recorded after placing them one at a time in the spectrometer. A line of best fit is produced in the graph with the uncertainties of slope and y-intercept shown. Lastly, the cuvette with the unknown concentration is put into the spectrometer and the absorbance is collected.

Observations No visual changes were noticed in the flask containing 35mL of unknown Mn 2+ concentration and 5mL of phosphoric acid after they were mixed as the solution stayed the same colour. Even after 0.4g of KIO 4 was added to the flask, the colour of the solution did not alter. During heating the solution in the hot water bath, it was seen that the solution became more and more purple slowly as time passed and by the end of 10 minutes, the colour was dark purple. Moreover, while making the other samples in the cuvettes, it was observed that the purple got darker in the sample as the concentration was increased, with the most concentrated solution having the darkest purple colour. It was also observed that the unknown solution had about the median colour of purple when it was compared with the samples.

Questions 1. Beer’s Law ( A x =ε x b C x Error: Reference source not found) shows a straight line equation when the absorbance versus concentration is plotted, as you did. Use the straight line fit on your graph to determine the extinction coefficient  for your compound. max=534.4 nm A= 0.751 C=4.415 x 10-4 =A/C =(0.751)/(4.415 x 10-4) =1701.02 M1cm-1 =1701.02(0.01840/0.751) =+41.68 Therefore, =1701.02+ 41.68 M-1cm-1

2. Use your measured absorbance to determine the concentration of the unknown solution. First calculate the concentration of the solution in the 100 mL volumetric flask, then, remember that you used 35 mL of the stock and diluted it to 100 mL, and calculate the concentration in the

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stock bottle. Use the uncertainties from the graph to find the uncertainty in your unknown solution concentration by propagation. 0.435=1701.02x + 0.02633 0.4087=1701.02x x= 2.40 x 10-4 x=2.40 x 10-4((67.06/1706)+(0.01840/0.02633)) x=+ 0.000177 3. List 3 sources of systematic error that you may have encountered in this lab. One of the possible sources of systematic error could be due to the cuvettes being contaminated which would affect the recorded absorbance by them, causing errors in the graph and hence the calculated concentration. Moreover, another source could have been due to the equipment making measurement and calibration errors, for example, there is some possibility that the toploading balances were not calibrated properly, causing the masses of the products to be measured incorrectly or the SpectroVis was not calibrated properly with distilled water, affecting absorbance readings. Lastly, the angle at which the absorbance was viewed and recorded could have varied when measuring for the different solutions, causing the value to be more or less than the true value and errors.

DATA SHEET

Re c or dt heunkno wn#f r o mt h e s t o c kbot t l ey ouus e d.

5 Es t i ma t e d Abs or banc e unc e r t ai nt yof a bs or banc e

St andar ds

Conc e nt r at i o n ont hebo t t l e

Conc e nt r at i o ni n Mol / L

1

2 4. 24

4. 415x10-4

0 . 7 84

+0. 01840

2

1 5. 15

2. 758x10-4

0 . 4 89

+0. 01840

3

3. 03

5 . 5 15x10-5

0 . 1 20

+0. 01840

4

9. 09

4 1 . 6 55x10

0 . 3 14

+0. 01840

Slope and Intercept (with uncertainties) from the Beer’s Law graph of your standard solutions.

m=1706+67.06 b=0.02633+0.01840

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Conc e nt r at i o nandunc e r t a i nt yi nt he Abs or banc e 10 0mLflas k( us ey ours l opeand ( unc e r t ai nt y) i nt e r c e ptf r om t hegr aph)

0. 43 5+0. 005

2. 40x1 0-4+0 . 0 00177Mol / L

Conc e nt r at i oni nt hes t o c k bot t l e :r e me mbe rt haty ou onl yt oo k~3 5mLf r om t he s t oc kbo t t l es or e s c al ey o ur 10 0mLt o35mL 6 . 86x10-4+0. 00 0177Mol / L

Calculated Concentration (and error propagated from the slope and intercept in the stock bottle in the units used thereon with uncertainties.

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References 1. Wu, Gang. Queen’s Chemistry First-Year laboratory Manual Chemistry 112; Queen’s University: Kingston ON, 2019-2020; p.69-72 2. Wu, Gang. Queen’s Chemistry First-Year laboratory Manual Chemistry 112; Queen’s University: Kingston ON, 2019-2020; p.72-76 3. Rachel Smith ( 20096341, [email protected]) Peng Zhang (20116364, [email protected]) Experiment 4 Spectroscopic Analysis using Beer’s Law. (Accessed November 2, 2018) 4. Venton, ffill. (2018). FfoVE. Retrieved from httpstt//www.love.com/science-education/10188/calibration-curves...