Wright CHM 1220 - CHM 1220_Exam3 Fall 2017_answers (3 pages)

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CHM 1220_Exam3 Fall 2017_answers



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CHM 1220_Exam3 Fall 2017_answers

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School:
Wright State University
Course:
Chm 1220 - General Chemistry II
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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 B Exam 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 2 pts b Calculate the pH prior to the addition of any NaOH solution C H O H O 10 Ka 8 pts 3 5 3 3 Because K is small x 0 575 M 3 86 x2 1 38 10 4 0 575 HC3H5O3 x x 1 38 10 4 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 x 8 91 10 3 M 0 575 x x H pH log 8 91 10 pH log H 3 pH 2 05 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 10 pts 10 mL soln HC3H 5O3 0 575 mol HC3H 5O3 0 00575 mol HC3H 5O3 initially 1000 mL soln 50 mL soln NaOH 0 115 mol NaOH 0 00575 mol NaOH added 1000 mL soln at equivalence point initial mol HC3H5O3 mol NaOH added OR 100 conversion to C3H5O3Need to write new equation to reflect what is happening Equilibrium for a base dissociation Kb HC3H5O3 OH C H O 3 5 3 1 10 14 7 25 10 11 4 1 38 10 0 00575 mol C3H 5O3 initial M C3H 5O3 0 0958 M C3H 5O3 initially 0 01 L 0 05 L x x 0 0958 x 7 25 10 11 Because K is small x 0 0958 M x2 7 25 10 11 0 0958 x 2 64 10 6 M x OH pOH log 2 64 10 pOH log OH 6 pOH 5 58 pH 14 pOH 14 5 58 pH 8 42 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 Reduction half reactions Ni2 aq 2e Ni s Ag aq 1e Ag s Eo 0 26 V smaller will be flipped as oxidation Eo 0 80 V larger will be reduction Electrochemical half reactions oxidation Ni s Ni2 aq 2e anode reduction Ag aq 1e Ag s cathode 2 pts o o o Ecell Ecathode Eanode 0 80 V 0 26 V 1 06 V b For this cell if constructed under standard conditions how much free energy is available to do work Need number of electrons involved in redox process Ni s Ni2 aq 2e2 Ag aq 1e Ag s net reaction 2Ag aq Ni s Ni2 aq 2Ag s n 2 mol e 2 pts 9 65 10 5 C o 1 06 V 2 05 105 J 205 kJ G ocell nFEcell 2 mol e 1 mol e c What is the equilibrium constant for this electrochemical reaction if under standard conditions 2 pts o nEcell 2 1 06 35 81 log K 0 0592 0 0592 K 1035 81 6 47 1035 d Write the balanced net ionic equation for this particular voltaic cell 2 pts 2Ag aq Ni s Ni2 aq 2Ag s 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 Non standard conditions need Nernst equation 2 pts Ecell Ecell Ecell 0 0592 0 0592 Ni 2 o E log Q Ecell log 2 n n Ag 0 02 0 0592 1 06 log 2 0 1 2 1 06 V 0 00891 V o cell Ecell 1 05 V 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 2 pts According to Le Ch telier 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


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