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EKC 108 TUTORIAL 2013/2014 1. The determination of Fe in an industrial waste stream was carried out by the o-phenanthroline. Using the data shown in the following table, determine the concentration of Fe in the waste stream. ppm Fe 0.00 1.00 2.00 3.00 4.00 unknown



Absorbance 0.000 0.183 0.364 0.546 0.727 0.269



2. A multiple standard addition method was used for determining Fe3+ in a natural water sample. 10 mL aliquots of the sample were pipetted into 50.mL volumetric flasks. Exactly 0.00, 5.00, 10.00, 15.00 ad 20.00 mL of standard solution containing 11.1 ppm of Fe3+ were added to each followed by an excess of thiocyanate ion to give the red complex Fe(SCN)2+. After dilution to volume, absorbances of the five solutions were measured in a 0.982 cm cell at 480 nm and found to be 0.240, 0.437, 0.621, 0.809 and 1.009 respectively. What is the concentration of Fe3+ in the water sample? 3. The concentration of Titanium and vanadium was determined through the formation of H2O2 complex. The absorbances of mixed solution Ti and V and standard solution 63.1 ppm Ti and 96.4 ppm V is given below: Wavelength 390 430 450 470 510



Std Ti 0.895 0.884 0.694 0.481 0.173



Absorbances Std V Mixed Solution 0.326 0.651 0.497 0.743 0.528 0.665 0.512 0.547 0.374 0.314



Calculate the concentration for Ti and V in the mixed solution. 4. To determine the formula for the complex between Fe2+ and ophenantroline, a series of solution was prepared in which the total concentration of metal and ligand was held constant at 3.15 x 10-4 M. The



absorbance of each solution was measured at a wavelength of 510 nm. Using the following data, determine the formula for the complex. XL 0.0 0.1 0.2 0.3 0.4 0.5



Absorbance 0.000 0.116 0.231 0.347 0.462 0.578



XL 0.6 0.7 0.8 0.9 1.0



Absorbance 0.693 0.809 0.693 0.347 0.000



5



6. The formula of Complex MXn can be determined. To 4 mL portions of a 0.01 M solution of M are added varying volumes of 0.01M solution of X, and the solutions are diluted to 20 mL with an appropriate background solution. The data are shown below No 1 2



mL of X 0 2



absorbances 0 0.125



3 4 5 6 7 8 9 10 11 12 13



3 4 5 7 8 9 11 12 13 14 15



0.185 0.248 0.310 0.435 0.498 0.559 0.685 0.738 0.771 0.775 0.775



7. The acidity constant for an acid-base indicator was determined by preparing 3 solutions, each of which has a total indicator conc of 5 x 10-5 M. The first solution was made strongly acidic with HCl and has an absorbance of 0.25. The second solution was made strongly basic with absorbance of 1.4. The pH of the third solution was measured at 2.91 with an abs at 0.662. What is the value of Ka for the indicator? 8. Why is a calibration curve likely to be linear over a wider range of concentrations at the wavelength of maximum absorption compared to a wavelength on a shoulder of the absorption curve? 9.



Distinguish between a single beam and a double beam spectrophotometer.



10.



Describe the factors that cause the Beer’s law relationship to depart from linearity.



11.



A second instrumental limitation to Beer’s Law is stray radiation. The following data were obtained using a cell with a pathlength of 1.00 cm, when the stray light is insignificant (Pstray = 0). [Analyte] (M) 0.000 0.002 0.004 0.006 0.008 0.010



Absorbance 0.00 0.40 0.80 1.20 1.60 2.00



Calculate the absorbance of each solution when P stray is 5% of P0 and plot beer’s law calibration curves for both sets of data. Explain any differences between the two curves. (Hint: Assume that P0 is 100) 12. A mixture of MnO4- and Cr2O72- and standards of 1.0 x 10-4 M KMnO4 and 1.0 x 10-4 M K2Cr2O7 gave the following results: Wavelength (nm) 266 288 320 350 360



MnO4- Standard 0.042 0.082 0.168 0.125 0.056



Absorbances Cr2O72- Standard 0.410 0.283 0.158 0.318 0.181



Mixture 0.766 0.571 0.422 0.672 0.366



Determine the molar concentration of each analyte in the mixture.



13. The acidity constant for an organic weak acid was determined by measuring a constant total concentration of the acid. Using the data from table. Determine the acidity constant for the organic weak acid. pH 1.53 2.20 3.66 4.11 4.35 4.75 4.88 5.09 5.69 7.20 7.78



Absorbance 0.010 0.010 0.035 0.072 0.103 0.169 0.193 0.227 0.288 0.317 0.317