1. From the emission spectra of quinine and anthracene, pick a wavelength that will permit you to determine quinine in a mixture at quinine and anthracene. Do the same for anthracene. Can you use the excitation spectra to distinguish between the two compounds?
Explain.
2. A solution contains 1.0 mg of KMn04/L. When measured in a 1.00 cm cell at 525 nm, the transmittance was 0.300. When measured under similar conditions at 500 nm, the transmittance was 0.350.
(a) Calculate the absorbance A at each wavelength.
(b) Calculate the molar absorptivity at each wavelength.
(c) What would T be if the cell length were in each case 2.00 cm?
(d) Calculate the absorptivity (if concentration is in mg/L) for the solution at each wave length.
3. An amount of 0.200 g of copper is dissolved in nitric acid. Excess ammonia is added to form Cu(NH3)42±, and the solution is made up to 1 L. The following aliquots of the solution are taken and diluted to 10.0 mL: 10.0, 8.0, 5.0, 4.0, 3.0, 2.0, and 1.0 mL. The absorbances of the diluted solution were 0.500, 0.400, 0.250, 0.200, 0.150, 0.100, and 0.050, respectively. A series of samples was analyzed for copper concentration by forming the Cu(NH3),21+ complex and measuring the absorbance. The absorbances were (a) 0.450, (b) 0.300, and (c) 0.200. What were the respective concentrations in the three copper solutions? If these three samples were obtained by weighing out separately (a)1000 g, (b) 2.000 g, and (c) 3.000 g of sample, dissolving
4. The following data were obtained in a external standard calibration for the determination of iron by measuring the transmittance, at 510 nm and 1.00 cm optical path. of solutions of nthrohnet to give a red-colored complex
(a) Calculate A, the absorbance, for each solution and plot A against concen-tration of iron. (You can do this using a spreadsheet program very easily.) Does the system conform to Beer's Law over the entire concentration range? (b) Calculate the average molar absorptivity of iron when it is deter-mined by this method. (c) Plot (100 - %T) against log concentration (Ringbom method). (1) What are the optimum concentration range and the ercent relative error per I % transmittance error) in maximum accuracy will the relative analysis this range? (2) Over what concentration rang error per 1% transmittance error not exceed 5%?
5. In prepanng a calibration curve for the determination of methyl ethyl ketone (MEK) solutions with different concentrations of MEK were prepared in chloroform. The absorbance at the Co. stretching frequency was measured.
The measured absorbance A for each solution is given. A blank (the pure solvent) had an absorbance = 0.00.
|
MEK concentration (%)
|
Absorbance
|
|
2
|
0.18
|
|
4
|
0.36
|
|
6
|
0.52
|
|
8
|
0.64
|
|
10
|
0.74
|
(a) Does the relationship between A and sample concentration deviate from Beer's Law? (b) Several unknown samples containing MEK were measured at the same wavelength as that used for the calibration curve. The results were as follows:
Sample Absorbance
A 0.27
B 0.44
C 0.58
D 0.69
What were the concentrations of MEK in the solutions A-D?