All questions are worth 25 points each. Show your work and circle your answers for full credit. OMIT ONE QUESTION by clearly writing OMIT in the space where you are to show your work, or by leaving the question COMPLETELY blank. Good luck.
1. A nitrate specific ion electrode is used to determine the nitrate concentration an industrial effluent. Standards prepared in a constant ionic strength buffer gave the following readings:
[NO3-] Potential (mV)
5.0 x 10-3 -107.6
1.0 x 10-2 -126.2
3.0 x 10-2 -137.5
5.0 x 10-2 -150.1
Calculate the slope, y-intercept, and the correlation coefficient for the linearly related data.
A 5.00 mL aliquot of unknown was diluted to 100.0 mL, and the potential of the solution was determined to be -128.4 mV. Calculate the concentration of the nitrate ion in the unknown.
3. Consider the simultaneous determination of compounds A and B. At 254 nm, A has =2500, while B has =150. At 322 nm, A has =275, while B has =1500. A mixture of A and B had an absorbance of 0.539 at 254 nm, while the same mixture had an absorbance of 0.610 at 322 nm. Calculate the concentration of A and B in the mixture, assuming all measurements were made in a 1.00 cm cell.
2. The drug tolbutamine (MW=270) has a molar absorptivity of 703 at 262 nm. One tablet is dissolved in water and diluted to a volume of 2.00 L. If the solution exhibits an absorbance in the UV region at 262 nm equal to 0.687 in a 1.00-cm cell how many grams of tolbutamine are contained in the tablet.
4. Calcium in a sample solution is determined by atomic absorption spectrophotometry. A stock solution of calcium is prepared by dissolving 1.834 g of CaCl2(H2O)2 (MW=129.01) in water and diluting to 1.000 L. A second solution (the stock solution) is then prepared by diluting 100.0 mL of this solution to 1.000 L. This second solution is used to prepare a set of calibration standards by taking 5.00, 10.00, and 20.00 mL of the solution and diluting to 100.0 mL. The sample is diluted by taking 10.0 mL of the solution and diluting to 250.0 mL. Strontium chloride is added to all solutions before dilution to avoid phosphate interference. A blank is prepared similarly. Absorbance signals on the stripchart recorder when the solutions are aspirated into an air-acetylene flame were as follows:
mL stock cm
0.00 1.5
5.00 10.6
10.00 20.1
20.00 39.2
Unknown 27.6
What is the concentration of the sample in ppm?
5. A sample containing silver is analyzed spectrophotometrically by AAS. A 1.500 g sample of the unknown was dissolved in a dilute nitric acid solution, which was filtered and the filtrate diluted to 500.0 mL in a volumetric flask. 10.0 mL of this solution was then diluted to 1.000 L in a volumetric flask to form the stock solution. When 25.0 mL of the stock solution was diluted to 50.0 mL an absorbance of 0.458 was measured. A second 25.0 mL aliquot of unknown was 'spiked' with 5.00 mL of 50.0 ppm silver solution and diluted to 50.0 mL. This second solution had an absorbance of 0.874. What is the percent silver in the original sample?
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