Questions How would the absorbance of the standard solution (tube 1) have been a
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Questions How would the absorbance of the standard solution (tube 1) have been affected if 0.30 M Fe(NO), had been used in place of 0.20 M? Remember that for the standard solution we have assumed that the reaction has been driven completely to FeSCN2. If it changes, calculate the new absorbance based on your values. 1. om aohe0 M Nasco bal beon wodin 2s o.IS place of 0.00040 M? If it changes, calculate the new absorbance based on your values. What would be the effect of improper rinsing of a cuvette prior to introducing a new solution? Considering the order of determination, how would the actual concentration of the FeSCN- in the cuvette differ from your sam ple in the test tube? 3. ru 4. The concentrations of Fe3+ and SCN are calculated by difference. In which tube is the concentration of Fe least accurately known? weoswrn 5. Assume that it is necessary to determine the [Fe*] in a solution that may be in the 0.0001 to 0.0005 M range. Outline a spectrophotometric method that would accurately give the required data. Be specific about the solu- tions that you would use. 6. Using your equilibrium constant, check the assumption that all of the SCN was converted to FeSCN2 in tube 1. Assume that the value of [FeSCN2 1 at equilibrium is an unknown quantity x. Express the values of [Fe] and SCN 1 at equilibrium based on the initial concentrations minus the amount that reacted to form [FeSCN2 Solve the quadratic equation for x, what fraction of thiocyanate ion is actually converted into the complex ion?Explanation / Answer
1. We first calculate the initial concentration of Fe3+, based on the dilution that results from adding KSCN solution and water to the original 0.20 M Fe(NO3)3 solution.
Calculating [Fe3+]i using the equation:
[Fe3+]i= (Fe(NO3)3 ml/total ml)*(0.20M)
[Fe3+]i= 0.10M
[SCN-] is calculated using the formula:
[SCN-]= (SCN ml/total ml)*(2*10-4)
[SCN-]= 1* 10-4M
Now, we calculate [FeSCN2+]:
[FeSCN2+] = 2.9*10-4M
So the equilibrium constant is Keq:
Keq = [FeSCN2+]/[Fe3+]i*[SCN-]
Keq = 14.5
Absorbance of [FeSCN2+] when Fe3+ is 0.20M
[FeSCN2+] = (1/10)*(0.20) = 0.020M
Absorbance of [FeSCN2+] when Fe3+ is 0.30M
[FeSCN2+] = (1/10)*(0.30) = 0.030M
2. As of the above case
We first calculate the absorbance of NaSCN at 0.00040M
[NaSCN]= (1/10)*(0.00040)= 0.000040M
Now absorbance at 0.00050M
[NaSCN]= (1/10)(0.00050)= 0.000050M
So it changes but to a very little extent.
4. The concentrations of Fe3+ and SCN- are calculated using the following formula:
for Fe3+ concentration at equilibrium
[Fe3+]eq= [Fe3+]i - [FeSCN2+]eq
Similarly for SCN-
[SCN-]eq= [SCN-]i - [FeSCN2+]eq
As we see the table,
We come to know that the concentration of Fe3+ is lowest in tube 5
Hence , it has least concentration of Fe3+.
5.
Spectroscopy:
Spectroscopy is the study of electromagnetic radiation. Visible light is one form of electromagnetic
radiation. Spectroscopy can be used for determination of the concentrations of solutions.
If a solution is colored, the intensity of the color is proportional to the concentration of the colored
solute.In its modern form, it is most commonly called Beer's Law. Its mathematical form is shown below-
A = ? l c
where,
A = absorbance, the amount of light absorbed by the sample.
? = molar absorptivity
l = path length, or distance the light travels through the solution
c = concentration of solute
It is important,to specify the wavelength of light at which the
measurements are made. In this experiment, we will work at 450 nm, red light at which the molar
absorptivity of FeSCN2+ is at its maximum.
Part A: Preparation of Standard Solutions and Beer’s Law Plot
1. Obtain 3 serological pipettes and a set of test tubes from the bench.
2. Set up a beaker for waste solution.
3. Obtain about 10 mL of 0.100M Fe(NO3)3 in 0.1 M HNO3 in a clean, dry test tube. Obtain about 10
mL of 5.00 x 10-4 M NaSCN in 0.1 M HNO3 in another test tube. Label the solutions.
4. Using pipettes, add the amounts of the Fe3+ solution, SCN- solution and water required for each
run to test tubes. Refer to Table 1 on your sheet(2nd image) for this. Use a stirring rod with a curved tip
to mix the solution until the color is uniform.
5. Pour the first solution from the test tube into a cuvette. Measure and record the absorbance of the solution. When the measurement is complete, pour the solution from the cuvette back into the test tube. Measure the absorbances of all the solutions in turn.
6. Using a graphing calculator or the spreadsheet function in the computer, plot the absorbance of
the FeSCN2+ solutions as a function of their concentrations.
7. If the plot is satisfactory(R2 should be greater than 0.95) empty cuvettes, rinse them, and
dry them as thoroughly as you can. If you have a point or two that don't fit the line, make sure the
color of the solution in question is reasonable. You may need to re-read an absorbance or remake
a standard to get a good calibration.
Part B: Preparation of the Equilibrium Mixtures and Absorbance Measurements
1. Obtain about 15 mL of 2.00 x 10-3 M Fe(NO3)3 in 0.1 M HNO3 and 15 mL of 2.00 x 10-3 M NaSCN
in 0.1 M HNO3 in clean, dry graduated cylinders that have been labelled.
2. Condition your pipettes with the new solutions of Fe3+ and SCN-.
3. Using pipettes, add the amounts of the Fe3+ solution, SCN- solution and water required for each
run to test tubes. Refer to Table 2 on your sheet(2nd image of question) for this. Use a stirring rod with a curved tip
to mix the solution until the color is uniform.
4. Measure and record the absorbance each solution as you did in Part A.
5. Use the absorbances for the solutions to determine the equilibrium concentration of FeSCN2+ for each run.
6. Fill in the reaction tables, and calculate Keq for each run. Then determine the average Keq, the
range of values, and the range as a percent of the average.
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