Final Exam with Formula Sheet | Analytical Chemistry | CHEM 2114, Exams of Analytical Chemistry

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Constants and conversion factors: c (speed of light) = 2.9979x108 m/s h (Planck’s constant) = 6.626x10-34 J·s F (Faraday constant) = 96,485 C/mol = 96,485 J/mol·V R (gas constant) = 0.0821 L·atm/K·mol = 8.3145 J/mol·K 1 atm = 760 Torr = 760 mm Hg = 14.7 psi = 1.01x105 N m-2 Avogadro’s number = 6.0221x1023 mol-1 Kw′ = 1.01x10-14 Absolute zero = 0 K = -273.15oC 1 L·atm = 101.3 J Some useful equations: E = hν c = λν A = abc or A = εbc A = -log(T) = -log(P/Po) 2 4 2 b b ac x a    ∆G = ∆Go + (RT)ln(Q) ∆Go = -(RT)ln(K) ∆G = -nFE and ∆Go = -nFEo E = Eo - (RT/nF)ln(Q) o 0.0592 V logE Q n E       (25 °C) cell cathode anodeE EE   measured test refE EV      ln ln a E k const RT   max s o M s v C v K C   2 x x 1 2 S z C    2 x x x 0.509 log 1 3.29 z S S     HA 3 3 3A H O OH H O H O OH a C K' C                          m+ x- 1 ( ) M X a b sp a b a b K s a b         ( ) 1 1 n orgA aq p aq V K nV           (Vorg and Vaq are total volumes of the organic and aqueous phases, respectively.) + 31 /[H O ] p C a K D K    (for a monoprotic weak acid) Eo (V) Eo (V) Ce4+(aq) + e- ƒ Ce3+(aq) 1.60 Cu2+(aq) + 2e- ƒ Cu(s) 0.337 MnO4- (aq) + 8H+(aq) + 5e- ƒ Mn2+(aq) + 4H2O 1.507 Sn4+(aq) + 2e- ƒ Sn2+(aq) 0.139 Cl2 (aq) + 2e- ƒ 2Cl-(aq) 1.36 2H+(aq) + 2e- ƒ H2 (g) 0.000 O2 (g) + 4H+(aq) + e- ƒ 2H2O 1.23 Sn2+(aq) + 2e- ƒ Sn(s) -0.140 Br2 (g) + 2e- ƒ 2Br-(aq) 1.087 V3+(aq) + e- ƒ V2+(aq) -0.255 Ag+(aq) + e- ƒ Ag (s) 0.779 Ni2+(aq) + 2e- ƒ Ni(s) -0.257 Fe3+(aq) + e- ƒ Fe2+(aq) 0.771 Fe2+(aq) + 2e- ƒ Fe(s) -0.440 reaction order linear plot 0 [A] vs. t 1 ln[A] vs. t 2 1/[A] vs. t Relative Polarity: aliphatic hydrocarbons < aromatic hydrocarbons < ethers < esters < ketones < aldehydes < amines < alcohols < organic acids < water CHEM 2114 - Analytical Chemistry Final Exam May 4, 2002 Instructions: Record your answers for questions 1-28 on the OPSCAN sheet using a number 2 pencil. Select only one answer from the given choices. Each multiple choice question is worth 4 points. Honor Code Statement: As stated on the syllabus, the Virginia Tech Undergraduate Student Honor Code applies to all work and exams in this course. 1. Which one of the following conjugate acid-base pairs is the best choice to prepare a buffer with a p[H3O+] of 5.0 ? a. choloracetic acid / sodium chloroacetate Ka = 1.36×10-3 b. formic acid / sodium formate Ka = 1.82×10-4 c. benzoic acid / sodium benzoate Ka = 6.28×10-5 d. acetic acid / sodium acetate Ka = 1.76×10-5 e. phenol / phenolate Ka = 1.05×10-10 f. ammonium chloride / ammonia Ka = 5.70×10-10 2. Which one of the acids listed in question 1 could produce the titration curve to the right ? 3. Predict the p[H3O+] of a 0.0800 M solution of sodium benzoate. (You may neglect activity effects and use the Ka listed above as the Ka´.) a. 2.87 b. 3.99 c. 5.45 d. 7.98 e. 8.55 f. 9.24 4. Predict the p[H3O+] of the resulting solution when 100 mL of 0.100 M NaOH is mixed with 200 mL of 0.100 M solution of ammonium chloride. (You may neglect activity effects and use the Ka listed above as the Ka´.) a. 2.87 b. 3.99 c. 5.45 d. 7.98 e. 8.55 f. 9.24 5. Which one of the following salts produces a basic solution when dissolved in water ? a. NaF b. NaCl c. NaBr d. NH4Br e. NaClO4 6. A reaction will always be spontaneous when: a. Ea is positive b. ΔG is positive c. ΔG is negative d. ΔH is positive e. ΔH is negative 7. How will increasing the ionic strength of a solution affect the formation constant for the complex Mg(OH)+(aq) ( o fK = 380)? (You may neglect competing equilibria and assume that αx is the same for all ions.) a. of fK K′ = b. o f fK K′ > c. o f fK K′ < 2 CHEM 2114 - Analytical Chemistry Final Exam May 4, 2002 20. An exponential function is common in science because it results from a direct proportionality to the amount of an analyte. One example is the decay of a radioactive material. Which of the following examples also involve an exponential decay ? a. The intensity of light as a function of path length through a sample. b. The absorbance, A, of light as a function of path length through a sample. c. The concentration of a reactant when the reaction has zero-order kinetics with respect to that reactant. d. The concentration of a reactant when the reaction has first-order kinetics with respect to that reactant. e. a. and b. f. a. and c. g. a. and d. You have the calibration curve to the right for the absorbance of riboflavin (vitamin B2) at 254 nm. 21. What is the riboflavin concentration in a solution that has an absorbance of 0.288 ? a. 0.280 mM b. 0.288 mM c. 0.556 mM d. 0.571 mM e. 0.771 mM 22. In another measurement, the measured solution came to 0.0532 mM. This sample originated from an analysis of a weight-loss tablet in which 100.0 mg of the tablet was dissolved in 100.0 mL of a dilute acid solution. If riboflavin has a formula weight of 376 g/mol, what is the weight percent of riboflavin in the tablet ? a. 0.550 % b. 1.75 % c. 2.00 % d. 3.50 % e. 5.32 % 23. Maximizing the extraction of an organic compound that contains an amine group with a pKa = 10.0 from aqueous solution requires that the aqueous solution have: a. p[H3O+] < 10.0 b. p[H3O+] > 10.0 c. p[H3O+] = 10.0 d. p[H3O+] = 7.0 24. Toxic metal ions such as Cd2+ are often complexed with natural humic acids in aquatic systems. How will the concentration of free metal ion change if the water system becomes more acidic ? (You may neglect changes in ionic strength due to the addition of acid.) a. [Cd2+] increases b. [Cd2+] decreases c. [Cd2+] does not change 5 y = 0.5047x + 0.0075 0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.2 0.4 0.6 0.8 1.0 Riboflavin Concentration (mM) A b so rb a n ce CHEM 2114 - Analytical Chemistry Final Exam May 4, 2002 25. The activation energy of digestion in a cold-blooded reptile is 55 kJ/mol. By what factor will the rate constant of the reaction catalyzed by this enzyme change if the body temperature of the animal decreases from T1 = 28 oC to T2 = 18 oC ? (Hint: remember to work in temperature units of K.) a. 0.033 b. 0.33 c. 0.47 d. 0.93 e. 1.33 26. For which one of the following insoluble salts will the solubility be most affected by the presence of NH3 ? The trend in Kf´ of the metal ammonia complex is: Mg(NH3)2+ < Zn(NH3)2+ < Cu(NH3)2+ < Hg(NH3)2+ (You may neglect the change in p[OH-] due to addition of NH3.) a. Mg(OH)2 b. Zn(OH)2 c. Cu(OH)2 d. Hg(OH)2 The figure to the right shows the effective formation constant for zinc and calcium EDTA complexes (plotted as log K’eff) as a function of p[H3O+]. 27. At what p[H3O+] can you measure Ca2+ in the presence of a comparable amount of Zn2+ using EDTA titration and keep the error introduced by the zinc interference to less than approximately 1% ? a. 8.0 b. 9.0 c. 10.0 d. 11.0 e. 12.0 28. The K’eff decreases at high p[H3O+] due to: a. EDTA being protonated. b. EDTA being deprotonated. c. T he metal ion forming complexes with protons. d. The metal ion forming complexes with hydroxide ions. e. Paranormal effects enhanced by the conjunction of five planets in the western sky at dusk. (This occurred during 2002. On May 6, 2003 I recommend looking for Mars before the sunrise.) 6