Beer\'s Law - Beer\'s Law Lab - Fall Semester Grade: A+ PDF

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Beer's Law Lab - Fall Semester
Grade: A+...

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Na me :

Pa r t ne r :

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Experiment 4: Beer’s Law. Purpose To determine the solute concentration of a solution through chemical analysis of spectroscopic techniques.

Introduction Beer’s law, or Lambert-Beer law was proposed by German chemist and mathematician August Beer in 1952. The law examines the absorption of radiant energy by an absorbing medium. The law states that, as the solute concentration of solutions increases, the absorption of radiant energy by the solution also increases in a linear trend1. The spectrometer was invented in 1859 by German chemist Robert Wilhelm Bunsen 2. It is a tool used to examine the absorption of solutions or other absorbing solid, liquid, or gas absorbing substances3. It does so by emitting continuous wavelengths of radiant energy (light) from a source into a substance and receiving the light that passes through the substance on the other side 3. The transmittance of a substance is then determined but the equation T = I/I0 where I0 is the incident light intensity and I is the final intensity3. Furthermore, absorption is determined by the equation A= -logT3.

Procedure For additional details, refer to Queen’s First Year Chem 112 Lab Manual3. 1. A Mn2+ of unknown concentration was used to prepare a permanganate solution. 2. A series of 4 KMnO4 solutions were used to determine max using SpectroVis apparatus and Logger Pro. 3. A Beer’s law correlation (absorbance (y-axis) vs concentration (x-axis)) was plotted for the 4 standard solutions using the SpectroVis apparatus and Logger Pro. 4. The absorbance of the ambiguous permanganate solution prepared in part 1 was determined using the SpectroVis apparatus and Logger Pro.

Observations 1. When H3PO4 is added to the Mn2+ solution, the solution becomes somewhat cloudy but still translucent. 2. The observed change above dissipates 3. When the catalyst is added (0.48g), the solute dissolves in the solution.

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4. When placed in the boiling water, the clear transparent solution becomes a purple translucent. 5. The deepness of the violet solution increases with the progression of heating. 6. max of the most concentrated standard permanganate solution is 496.8 nm 7. Absorption of unknown permanganate solution is 0.350(2)

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.

Since Ax = εxbCx

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Since b = 1.00 cm Therefore Ax = εxCx Therefore, εx= Ax/Cx = Slope of Beer’s Law curve (m) = 936.6 M -1cm-1 ± 107.8 M-1cm-1 Unit derived from εx = Ax/Cxb where Cx is in M and b is in cm max = 496.8 nm 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, calculate the concentration in the stock bottle. Use the uncertainties from the graph to find the uncertainty in your unknown solution concentration by propagation. Since Ax = mCx + b b (intercept) = 0.01953 ± 0.02958 Cx = (Ax – b)/m = (0.350 - 0.01953)/936.6 = 3.5284 × 10-4 M Concentration in stock bottle = (100 ml/35 ml) (3.5284 × 10-4 M) = 1.01 × 10-3 M Uncertainty of Cx = 3.5284 × 10-4 ((0.002 + 0.02958/0.350 - 0.01953) + (107.8/936.6)) = 7.4329 × 10-5 M Uncertainty of concentration in stock bottle = (100 ml/35 ml) (7.4329 × 10-5 M) = 2.12 × 10-4 M Concentration in stock bottle = 1.01 × 10-3 M ± 2.12 × 10-4 M

3. List 3 sources of systematic error that you may have encountered in this lab. 1. Some of the phosphoric acid used in preparing the permanganate solution in part 1 may have stuck to the beaker and not dispensed into the manganate solution to undergo a reaction. Thus, the final concentration of permanganate ions may have been slightly reduced, reducing the final absorption value for the unknown solution. 2. The lab manual used indicates that the spectrometer devices used, SpectroVis, were lacking in accuracy3. Therefore, the absorption values obtained with respect to the concentrations, as well as the max of the solutions were possibly lacking in accuracy, deviating above or below the actual values. This impacts the final concentration calculation of the unknown solution, as the correlation obtained and used for it may be faulty 3. Another possible source of error is that the cuvettes, although washed and wiped clean before the experiment, may not have been 100% clean. Any form of contamination on the cuvettes surface may would have reduced the amount of light that passed through the solution and resulted in an absorbance value higher than what the solution actually yields.

DATA SHEET

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

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St andar ds

Conc e nt r at i o n ont hebo t t l e ( mg/ mL)

Conc e nt r at i o ni n Mo l / L

1

9. 09

1. 65×1 0-4

0 . 1 89

2

1 5. 1 5

2. 76×1 0-4

0 . 2 41

3

3. 03

5. 52×1 0-5

0 . 0 77

4

2 4. 2 4

4 4. 41×1 0

0 . 4 50

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

Abs or banc e ( unc e r t ai nt y)

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

5 3. 53×1 0-4M ±7 . 43×1 0 M

±0 . 001 ±0 . 001 ±0 . 002

Slope = 936.6 M-1cm-1 ± 107.8 M-1cm-1 Intercept = 0.01953 ± 0.02958

Conc e nt r at i o ni nt hes t o c k bot t l e :r e me mbe rt ha ty ou onl yt oo k~3 5mLf r o mt he s t o c kbo t t l es or e s c al ey o ur 10 0mLt o3 5mL

Conc e nt r at i o nandunc e r t ai nt yi nt he 10 0mLflas k( us ey ours l opeand i nt e r c e ptf r o mt hegr a ph)

0. 350( 2)

±0. 0 01

1. 01×1 0-3M ±2 . 12×1 0-4M

References 1. Rafferty, J. P. (2018, March 6). Beer's law. Retrieved from https://www.britannica.com/science/Beers-law.

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2. ASTROLab du parc national du Mont-Mégantic. (n.d.). Spectrometers: Instruments. Retrieved from https://astro-canada.ca/les_spectrometres-spectrometers-eng. 3. Queen’s Chemistry, First-Year Laboratory Manual, Chemistry 112, Pages 69-76...