A FIGURE 222 Relationship between absorbanoe and conoentration acoording to the
ID: 502677 • Letter: A
Question
A FIGURE 222 Relationship between absorbanoe and conoentration acoording to the this law occur, it is wise to construct a calibration curve of ab- versus concentration. A. Preparation of the Calibration Curve You will measure the absorbance of a series of solutions with known FeNCS ion concentrations to prepare a calibration curve. The experimental approach to this an interesting dilemma. The reaction that you will be using to pre- involves pare these solutions (reaction [6D is reversible and does not proceed to comple- tion. How then can solutions with known FeNcs2* concentrations be prepared? You will take advantage of Le Chatelier's principle to shift the equilibrium of [6] to the right section 15.7). When you make an equilibrium mixture in which the Fe' ion concentration is relatively high, the equilibrium expression (see Example indicates that the scN concentration for this mixture is rela tively low. Consequently, by preparing a solution in which the initial Fe ion concentration is much larger than that ofthe SCN ion, you can force the reaction to proceed nearly to completion. Under these conditions, you will assume that all of the scNT is in the form of FeNcs? GIVE IT SOME THOUGHT when a high concentration of Fe is used, which way will the equilibrium shift? Label six 50.0 mL volumetric flasks 1 through 6. Pipet 10.00 mL of 200x10 M FeNOs) solution into each volumetric flask. Then pipet 1.00, 2.00, 3.00, 4.00, and 5.00 mL of 2.00x10 3 M NascN solution into flasks 2 through 6, respectively. (Caution: Nitric acid and solutions containing it can cause burns and skin discoloration. you come into contact with it, Instructor: students can save time in thsexperiment if you set up several burets filled with Fo(NO), Nasor, and HNo, olutions on side tables in the laboratory. Suudents may obtain these solutions from the burets and refill the burets from bottles ofstock solutions also localed there. Copyright c2012 Pearson Education, Inc.Explanation / Answer
For the given reaction,
Fe3+ + SCN- <==> [FeNCS]2+
Formula,
[Fe3+]initial = molarity of [Fe3+] x volume of Fe3+ solution taken/total volume
[SCN-]initial = molarity of [SCN-] x volume of SCN- solution taken/total volume
For A, [SCN-]initial = [FeNCS]2+
Plot, Absorbance vs [FeNCS]2+
slope = molar absorptivity
Equilibrium,
[FeNCS]2+eq = absorbance/slope
[Fe3+]eq = [Fe3+]initial - [FeNCS]2+eq
[SCN-]eq = [SCN-]initial - [FeNCS]2+eq
Keq = [FeNCS]2+eq/[Fe3+]eq.[SCN-]eq
absorbance = 2 - log(%T)
A. Calibration
[Fe3+] molarity = 0.2 M
[SCN-] molarity = 0.002 M
Tube [Fe3+] (ml) [SCN-] (ml) [FeNCS]2+ (M)
1 10 0 0.002 M x 0 ml/50 ml = 0
2 10 1 0.002 M x 1 ml/50 ml = 4 x 10^-5
3 10 2 0.002 M x 2 ml/50 ml = 8 x 10^-5
4 10 3 0.002 M x 3 ml/50 ml = 12 x 10^-5
5 10 4 0.002 M x 4 ml/50 ml = 16 x 10^-5
6 10 5 0.002 M x 5 ml/50 ml = 20 x 10^-5
Plot Absorbance vs [FeNCS]2+
slope = molar absorptivity = (0.220 - 0.156)/(12 x 10^-5 - 8 x 10^-5) = 1600 M-1.cm-1
B. Equilibrium constant
[Fe3+] = 0.002 M
[SCN-] = 0.002 M
Tube 1 2 3 4 5 6
Absorbance 0 0.214 0.474 0.780 0.860 0.959
Tube [Fe3+]initial (M) [SCN-]initial (M) [FeNCS2+]eq (M) [Fe3+]eq (M) [SCN-]eq Keq
1 0.001 0 0 0.001 0 0
2 0.001 2 x 10^-4 1.34 x 10^-4 8.66 x 10^-4 6.60 x 10^-5 2344
3 0.001 4 x 10^-4 2.96 x 10^-4 7.04 x 10^-4 1.04 x 10^-4 3003
4 0.001 6 x 10^-4 4.87 x 10^-4 5.13 x 10^-4 1.13 x 10^-4 8401
5 0.001 8 x 10^-4 5.37 x 10^-4 4.63 x 10^-4 2.63 x 10^-4 4410
6 0.001 10 x 10^-4 6.00 x 10^-4 4.00 x 10^-4 4.00 x 10^-4 3750
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