CHEMISTRY 226 EXAM 3 NOVEMBER 21, 2002 Name: Student ID No.: I. (2 pts. each) Match the following terms and definitions. 1. ____ When the amount of added titrant is chemically equivalent to the amount of analyte in the sample. 2. ____ The physical change associated with the condition of equivalence that we can observe. 3. ________ Added to the analyte solution to give an observable physical change at or near the equivalence point. 4. _______ Solution consisting of a conjugate acid/base pair formed when a weak acid is titrated with a strong base. 5. _______ Nine or more atoms in a cycle that form a three dimensional cavity that can just accept an appropriately sized ion. 6. _______ Compound whose purity has been established by chemical analysis and serves as a reference. 7. _______ The number mols of strong acid or base that causes 1L of the buffer to undergo 1 unit change in pH. 8. _______ pKa ± 1 A. Buffer G. Indicator B. Buffer Capacity H. Indicator pH range C. Chelate I. Ligand D. Coulometric Titration J. Linear Segment Curve E. End Point K. Macrocycle F. Equivalence Point L. Primary Standard M. Secondary Standard II. (2 pts) How many binding sites does EDTA have? ________ (2 pts) How many Al3+ ions bind/molecule EDTA?_________III. (8 pts). A 4.476 g sample of petroleum is burned and the SO2 produced collected in 3% H2O2 in the following reaction: SO2 + 2 H2O2 Æ H2SO4 A 250 mL portion of 0.00923 M NaOH was introduced into the solution of H2SO4 following which excess base was back-titrated with 13.33 mL 0.01007 MHCl. Calculate the ppm sulfur in the sample. IV. (8 pts) Calculate the pH of a solution that results from mixing 20 mL 0.200 M HCl with 25 mL 0.232 M NaOH. V. (10 pts) What is the pH of a solution prepared by dissolving 9.2 g lactic acid (90.08 g/mol) abd 11.5 g sodium lactate (112.06 g/mol) in water and diluting to 1.00 L. Ka = 1.38 X 10-4.VI. (10 pts) A 0.5843 g sample of plant food was analyzed for N content by the Kjeldahl method. The liberated NH3 was collected in 50.00 mL 0.1062 M HCl and the excess acid was back-titrated with 11.89 mL 0.0925 M NaOH. Express the results of the analysis in terms of % nitrogen and % protein. VII. (10 pts) A NaOH solution was 0.1019 M immediately after standardization. Exactly 500.0 mL of the reagent was exposed to air and absorbed 0.652 g CO2. Calculate the relative carbonate error in the determination of acetic acid using phenolphthalein as an indicator. VIII. (8 pts) List four requirements of primary standards.IX. (4 pts each) Write mass balance equations for a solution that is: 1. 0.10 M in Na2HPO4 2. A solution that is saturated with MgCO3. X. (4 pts each) Write charge balance equations for a solution that is: 1. 0.10 M in Na2HPO4 2. A solution that is saturated with MgCO3 XI. (10 pts) Calculate the pH of the solution that contains 0.240 M H3PO4 and 0.480 M NaHPO4. Ka= 7.11 X
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