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Wright CHM 1220 - CHM 1220_Exam3 Fall 2017_answers

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Exam #3 Multiple Choice Answers 1. D 2. D 3. C 4. C 5. D 6. B 7. A 8. C 9. B 10. C 11. B 12. A 13. B 14. A 15. BExam #3 Written Problem Solutions 16. Lactic acid is a common by-product of cellular respiration and is often said to cause the “burn” associated with strenuous activity. Consider a 10-mL sample of 0.575 M lactic acid (HC3H5O3, pKa = 3.86) that is titrated with 0.115 M NaOH solution. (a) Write an appropriate chemical reaction for the solution prior to the addition of any NaOH. (b) Calculate the pH prior to the addition of any NaOH solution. (c) What is the pH of the solution that results once 50 mL 0.115 M NaOH solution has been added? Is this at, before, or after the midpoint of the titration? initially OHHC mol 00575.0soln. mL 1000OHHC mol 0.575OHHC soln. mL 01353353353added NaOH mol 00575.0soln.mL1000NaOH mol 0.115NaOH soln. mL 50 2 pts at equivalence point: initial mol HC3H5O3 = mol NaOH added OR 100% conversion to C3H5O3- Need to write new equation to reflect what is happening: 86.33533353a10OHHCOH OHCK41038.1575.0  xxxBecause K is small, x < 0.575 M 421038.1575.0xMx31091.8 Hx05.2pH1091.8logpHHlogpH38 pts Equilibrium for a base dissociation: 11414353353b1025.71038.1101OHCOH OHHCKinitially OHC 0958.0L 0.05 L 0.01OHC mol 0.00575OHC initial-353353353MM 111025.70958.0  xxx1121025.70958.0xMx61064.2 OHx8.42pH58.514pOH14pH58.5pOH1064.2logpOHOHlogpOH6Because K is small, x < 0.0958 M 10 pts NOTE: Ka is actually large enough that the last decimal place in the initial concentration would be affected, so for the best answer the quadratic equation should be used. However, the ultimate pH would not be changed from that listed here using this approximation.17. An electrochemical cell is constructed such that a nickel electrode is immersed in a solution of Ni2+ ions and connected to a silver electrode immersed in a solution of Ag+ ions. Answer the questions that follow regarding this cell. A table of standard reduction potentials is provided on the back of the equation sheet. (a) What is the standard cell potential for this voltaic cell? (b) For this cell, if constructed under standard conditions, how much free energy is available to do work? (c) What is the equilibrium constant for this electrochemical reaction if under standard conditions? (d) Write the balanced net ionic equation for this particular voltaic cell: (e) If the cell is instead constructed at 298 K, using 0.02 M NiCl2 and 0.1 M AgNO3 solutions (and an appropriate charge-balancing electrolyte), what is the cell potential that would be measured under these conditions? BONUS: Using chemical principles, explain why sour milk, which contains lactic acid, may lead to tooth decay for young children if not removed. Apatite, Ca5(PO4)OH, is the mineral in teeth. It breaks down according to the following equation: Ca5(PO4)OH(s) ⇌ 5 Ca+ (aq) + 3 PO43- (aq) + OH- (aq) According to Le Châtelier’s principle, lactic acid will “consume” the OH- ion, thus driving the given reaction to the right. This will lead to further breakdown of apatite and therefore tooth decay. Ni2+(aq) + 2e-  Ni(s) Eo = -0.26 V Ag+(aq) + 1e-  Ag(s) Eo = 0.80 V Reduction half-reactions: (larger, will be reduction) Ni(s)  Ni2+(aq) + 2e- Ag+(aq) + 1e-  Ag(s) (smaller, will be flipped as oxidation)Electrochemical half-reactions: (oxidation) (reduction) (anode) (cathode)V 06.1V 26.0V 80.0oanodeocathodeocell EEE Ni(s)  Ni2+(aq) + 2e- 2(Ag+(aq) + 1e-  Ag(s)) 2Ag+(aq) + Ni(s)  Ni2+(aq) + 2Ag(s) Need number of electrons involved in redox process:net reaction n = 2 mol e-  kJ 205J 1005.2V 06.1e mol 1C 1065.9e mol 2G55ocellocellnFE2Ag+(aq) + Ni(s)  Ni2+(aq) + 2Ag(s) V 05.1V 0.00891V 06.10.10.02log20.059206.1AgNilog0.0592log0.0592cellcell2cell22ocellocellcellEEEnEQnEENon-standard conditions, need Nernst equation: 2 pts  81.350592.006.120592.0logocellnEK3581.351047.610 K2 pts 2 pts 2 pts 2 pts +2


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Wright CHM 1220 - CHM 1220_Exam3 Fall 2017_answers

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