Lab 2 - Chem Eq. & Beer's Law

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Objective:
To determine the concentration and equilibrium constant for a reaction of colored product using absorption spectroscopy, to create a calibration curve for determining equilibrium concentrations by applying Beer’s law and to force the completion of a reaction by applying Le Châtelier’s principle.

Procedure:
A diluted solution was made from the stock of 0.0025 M Fe(NO3)3 by diluting 4.0 mL to exactly 100.00 mL, using serological pipet and volumetric flask. The concentration of the diluted solution was calculated and recorded. The following known solutions were made up in labeled medium test tubes, using a serological pipet, mixing them thoroughly. For Part A: Absorbance of {Fe(SCN)}2+ “Knowns”

Table 1: Solution Mixtures to determine Absorbance of {Fe(SCN)}2+ “Knowns.” Test Tube| Diluted Fe3+ (mL)| Stock 0.50 M KSCN (mL)| Stock of 0.1 M HNO3 (mL)| 1| 1.0| 5.0| 4.0|
2| 2.0| 5.0| 3.0|
3| 3.0| 5.0| 2.0|
4| 4.0| 5.0| 1.0|
5| 5.0| 5.0| 0.0|

For Part B: Absorbance of {Fe(SCN)}2+ “Unknowns”
Test Tube| Stock 0.0025 M Fe(NO3)3 (mL)| Stock of 0.0025 M KSCN (mL)| Stock of 0.1 M HNO3 (mL)| 6| 1.0| 1.0| 5.0|
7| 1.0| 1.5| 4.5|
8| 1.0| 2.0| 4.0|
9| 1.0| 2.5| 3.5|
10| 1.0| 3.0| 3.0|
11| 2.0| 1.0| 4.0|
12| 2.0| 1.5| 3.5|
13| 2.0| 2.0| 3.0|
14| 2.0| 2.5| 2.5|
15| 2.0| 3.0| 2.0|

A Vernier SpectroVis Plus was attached to a LabQuest device to measure the absorbance. The spectrometer was calibrated with a cuvette filled with deionized water as a “blank.” The absorbance of the water and cuvette were stored in the spectrometer’s memory and subtracted from subsequent runs. An absorbance spectrum was acquired over the visible wavelength range of the most concentrated sample of {Fe(SCN)}2+. The cuvette was filled with the solution until ¾ full. The clear sides of the cuvette were cleaned after each use. The wavelength where the absorbance is at maximum, λmax was determined by taping the peak with the stylus. The absorbance of each solution of {Fe(SCN)}2+ at λmax was measured and recorded. All the solutions were discarded into the labeled waste container.

Data / Results:
A. Absorbance of {Fe(SCN)}2+ “Knowns”
M1V1 = M2V2 0.0025(4) = M2 (100.0)
M2 = 0.0001 M
M2 = [Fe3+]
Fe3+ (aq) + SCN- (aq) {Fe(SCN)}2+ (aq)

Table 3: Concentrations of {Fe(SCN)}2+ Part A.
| Test
Tube| Diluted Fe3+ (mL)| Stock 0.50 M KSCN (mL)| Stock 0.1 M HNO3 (mL)| Total Volume (mL)| Concentration of {Fe(SCN)}2+ (M)| | | 1| 1.00| 5.00| 4.00| 10.0| 0.00001| |
| 2| 2.00| 5.00| 3.00| 10.0| 0.00002| |
| 3| 3.00| 5.00| 2.00| 10.0| 0.00003| |
| 4| 4.00| 5.00| 1.00| 10.0| 0.00004| |
| 5| 5.00| 5.00| 0.00| 10.0| 0.00005| |

Table 4: Concentrations of {Fe(SCN)}2+ and Absorbance Part A. Concentration of {Fe(SCN)}2+ (M)| Absorbance|
0.00001| 0.145|
0.00002| 0.230|
0.00003| 0.304|
0.00004| 0.456|
0.00005| 0.466|

λmax for Test Tube 5 = 477nm

Graph 1: Absorbance vs. Concentration of {Fe(SCN)}2+

Slope = 8.68 X103 cm-1M-1
Intercept = 0.0598
R2 = 0.956

Molar Absorptivity = Slope = 8.68 X103 cm-1M-1

B. Absorbance of {Fe(SCN)}2+ “Unknowns”
A = εbc
ε = 8.68 X 103 cm-1 M-1

Table 5: Absorbance and Concentration of {Fe(SCN)}2+ Part B. Test
Tube| Stock 0.0025 M Fe3+ (mL)| Stock 0.0025 M SCN- (mL)| Stock 0.1 M HNO3 (mL)| Total Volume (mL)| Absorbance| Concentration of {Fe(SCN)}2+ (M)| 6| 1.00| 1.00| 5.00| 7.00| 0.0840| 9.68 X 10-6|

7| 1.00| 1.50| 4.50| 7.00| 0.158| 1.82 X 10-5|
8| 1.00| 2.00| 4.00| 7.00| 0.180| 2.07 X 10-5|
9| 1.00| 2.50| 3.50| 7.00| 0.320| 3.69 X 10-5|
10| 1.00| 3.00| 3.00| 7.00| 0.338| 3.89 X 10-5|
11| 2.00| 1.00| 4.00| 7.00| 0.360| 4.15 X...
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