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Spring 2011 A.pdf - PROBLEM 1(Solution Concentration...

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Unformatted text preview: PROBLEM 1 (Solution Concentration) INITIALS: MD (8 points) Citric acid is commonly used in food manufacturing especially candies and sofi drinks. It is also used as a cleaning agent in many household cleaning products. ' SUGGESTED TIMING FOR THIS ENTIRE PROBLEM: 6 MINUTES OR LESS ’ No credit will be given for an illegible answer or illegible work. Answer the following questions: (i) A typical household cleaner contains 6.05% (w/v%) of citric acid. Convert the w/v% to molarity. Moleculgrrygeightofcigjg gold is 192.12g/mol Show all your war]; or part (1) in the box provided below. (ii) Citric acid is a tri-protic acid (i.e. contains three acidic protons). Suppose during a reaction between NaOH and citric acid, only two acidic protons were involved in the reaction. Based on this information, convert your answer in part (i) to normality. Show all your work Yoffifitfllearlrwrite"out'alhhe'co -. , . the calculflign,/~v/‘ . CHEM 1 4BL_A_SI 1 PROBLEM 2 (Spectrophotometric Analysis) INITIALS: M2 (12 points) A student in biology performedza Beer’s law analysis experiment in the ultraviolet region (200-400nm) to determine the experimental molar absorp‘fivity constant for DNA. During the analysis, the student prepared five standard solutions that fell within the Beer’s law limit. The experimental data for all standard solutions also obey Beer’s law. In addition, the student prepared the appropriate and correct blank solution for the analysis. The student then constructed the Beer’s law calibration graph. SUGGESTED TIMING FOR THIS ENTIRE PROBLEM: 7 MINUTES OR LESS ’ No credit will be given for an illegible answer or illegible work. (i) Explain how the student can determine the experimental molar absorptivity constant for the DNA based on the *w—r—m analysis? DO NOT simply write outjusejnafliein'aficTéq uat1on as your explanation—e. ” WW Ex lanatioii ENTERCE I \ AP= etc (Ho—pf: 8L gigsg‘fjm‘éi' mSWWIOFXOJ 1‘0 CDRWWO’T ‘4 “M" So Slime: 8L pecau. s by (45mg Hm “Ops ' ' ' tum of Hm OlI’Ui'S {-— valcm Far 1'... - sbp-e/ L. um be. Gan/(ll ' (ii) There IS a n - - - a "9 ~ ': " ‘ = ‘ absorptivity constant based on the Beer’s law calibration graph constructed by the students. Based on the information provided 1n this question and assuming that the Beer’s law calibration line 15 correct, what would be a reasonable scientific explanation of why there IS a large deviation between the literature molar absorptivity constant and the experimental molar Myabsorptivity constant? Didi“ 11% max M75? rbmee w””“‘”"”” “M " ”WWW“ “Wren-Wm.“ Explanation ONE SENTENCE . " WE?) (L tar 0n bd- imagine v Q expermmnfisl 6, 14" Hm We wig/flwimu/n abs rb ca was not £1541 , . trio} tower ?\ L00 dweid at less sleep slow sen/LU), S‘I’Opa =~ 81—- (L con3+<int V ' -—~- BELOW THIS LINE -___.........____._.____ CHEM14BL_A_SII } A: 3 675‘“ .1 I PROBLEM 3 (Mixed Concepts) (15 points) Answer the following questions. SUGGESTED THVIING FOR THIS ENT ‘ No credit will be given for em illegible answer or illegible work. based on the following data. Show all your work (i) Calculate the % inherent error in the weight of MO; / Weight of the empty weighing boat: 0.20179 " Weight of the weighing boat + KMnO4: 0.76839 b ances listed on the Eggpage-é‘vu'ne’ea’t; be carefid of which one you dee‘ide'to use for them error analysis. /' a —N Vsolute uncertainties are listed on the cover a e o the exam There are TWO different types of digital (ii) Calculate the absolute error inm 01? of KMnO4 for the following procedure. Show all your work. 2. 0005M:I:0.0006M KMnO4 solution using a volumetric pipet / f Viv; szz. utmost/I momma: "A ”Liege 00mg 0 n00lmg} y 0 out. ‘(Zfiflgmol . 1. 0° 902.40%) :1 000053111 ’ /*"" ”hhuni we“ (1'1;an 1n 16W ' ' f a best buffer solution in terms a ratio between a weak acid and its can 'u ate base. Limit 0 __N_E_ sentence. 3' new '5 aim equjtv I v01\‘\ who bet/ a. mean 01313028qu can)» u ' Qfiifi MLUHWKQ}; m ‘ ‘3 ‘ ”' . 3 " II‘KW (iii) Using the concepts of ac'm“ :- ea. CHEM 14BL'_A_Sl 1 PROBLEM 4 (Acid-Base Equilibrium) INITIALS: MD (25 pointS) Shown below are the titration graph and the experimental titration set up between a weak base and a stron mono rotic acid. . , _ . ~ _ mm 8 p on we ’r H“ «A we r macaw DH ‘02‘112- 9’4“.“ mg: [LI die /x my“ m Volw'netric Buret a ‘/ Strong Monoprctic Acid Solution Beaker Volume of the Strong Monoproiic gafga“ / Acid Solution (ML) SUGGESTED TflVflNG FOR THIS ENTIRE PROBLEM: 14 MINUTES OR LESS fl No credit will be given for an illegible answer or illegible work. Answer the following questions. (i) Based on the concepts on acid-base equilibrium we discussed in lectures, explain what equilibrium determines the INITIAL pH on the titration graph? DO NOT use chemical equations or symbols (i.e. HA or A') in your answers. DO NOT simply write out mathematical equations as your answers. Lmit your explanation to ONE sentence. )) (ii) Based on the concepts on acid—base equilibrium we discussed in lectures, answer the following questions for the region between a and b on the titration graph. DO NOT use chemical equations or symbols (i.e. BA or A') in your answers. hm reg re a Which reactant is the limiting reagent? ONLY ONE chemical species. Answer: flag: I (l (I Lid q’ Which reactant is in excess? ONLY ONE chemical species. Answer: 11%; ]h We (iii) In TWO STEPS, outline how you can use the titration graph to determine tremagperimental weak basepolrw dissociation constant (pK ). Limit to QNj sentence per ste . 1" M I W‘VL‘I Iran cc soelu’mmb-m on" wnwmami’n Wm wig; equawga poms, where, Step “tweak base] *5 my: acid] . . ow 0 ‘ w ~ r. . Step#2:3 aim hill EQUJVWCQ Poihiggou CCU] Alumna Wm, (’58:, 060311-54 ...._.__.__._............_......... N0 ENTRY BELOW THIS LINE ———————-———-——-—-—7)——— 90H 3 9K Kb: dissociation 0? mail W \0 CHEMI4BL__A_Sr/ PROBLEM 4 (Acid-Base Equilibrium) INITIALS: MD (iv) Based on the concepts on acid-base equilibrium we discussed in lectures, explain what equilibrium determines the EQUIVALENCE POINT pH on the titration graph? Lmit your explanation to ONE sentence. DO NOT use chemical equations or symbols (i.e. HA or A') in your answers. DO NOT simply write out mathematical equations as your answers. Answerzl’I MI SIS Q WIIbrIWTI 0; 661?? W W WVWCQ/ omk— . (v) QN'hich chemical species determines the equilibrium pH on the titration graph between region c and d? Explain your reasoning in ONE sentence. DO NOT use chemical equations or symbols (i.e. HA or A') in your answers. DO NOT simply write out mathematical equations as your answers. Chemical Species that determines the equilibrium pH (ONLY ONE chemical species.): 5 ‘ 0 LLCIOL . NE (guess $0.1 maul M30 W0 Ex Ianation O Sentence : . r ‘ .9 p. ( ) Lela because "m“ P ab - 01‘ 0h I5_ C _ ns mi» - {“0 ESWGIM mad? I MW We base) Iiigfig 3mm mp . . . .i - .. o . r 0""? 2' " ‘5 '0' (vi) Explain why the pH in region 1: decreases much more rapidly on the titration graph as compared to y a region between a and b where the pH shows a relatively slow decrease in pH. DO NOT use chemical equations or symbols (i.e. HA or A') in your answers. DO NOT simply write out mathematical equations as your answers. {‘3 not w/m W barter region (Imam vegan mi/ (1 erg/50 W Mdgfibn ofisrnmg gig m reg/0n x 16 ' WSiS to H c ' x dzcmases mate , -...————--.—-——.——-—-—--——- NO ENTR BEL’ow _ -—————- -- __...__....._ .. ._.... W! E Explanation (ONE Sentence): @910“ X CHEM14BL_A_'SI 1 PROBLEM 5 (Scientific Graphing & Beer’s Law) INITIALS: M D (15 points) Carefully examine the Beer’s law calibration graph shown below for the analysis of the blue food-dye at 622nm. The “Abs.” notation on the graph corresponds to the various absorbance readings collected in the experiment. The ‘Conc.” notation on the graph corresponds to the concentrations for the various solutions containing the blue food—dye. For this uestion ou will assume that all the data oints shown below fall within the Beer’s law a roximation. Absorbance Abs. 4 ”Mm-MK} Abs- 3 Abs. 2 Abs. 1 ’ 0 Concentration (M) Conci Cone. 2 Cent. 3 Cone. 4 SUGGESTED TINIING FOR THIS ENTIRE PROBLEM: 10 MINUTES OR LESS “ No credit will be given for an illegible answer or illegible work. (i) Draw a best-fit line through the data points on the graph. (ii) Explain your rationale for the best-fit line that you completed in part (i). Limit to ONE sentence. 4 it: Answer: MWMWHTiZ-ésflfiliw ill/W fl I‘QJi’IOl’lS Hug ddfdfmoiniy will} _ 0m M Ma (imam, POlni‘S 3 LiOSve/ ‘ m rim as/ggfibta) (imxamine Whmfimrdwmpmmmmmndammbbmhatmfifihe Beer’s law calibration graph for the line that you drew in part (i)? If no, simply write “None” as your explanation. If yes, explain what the fundamental problem is in Beer’s law calibration graph for the best-fit line that you drew in Part (D 1/“ ([94 i W A F d talP bl S N0: 9 ' ny un amen ro em(YE or ) ., ll: M Explanation (ONE SEN x i. - <, ,_ m9- '9', O , U)“ 0' “ u 5 l . I' « . 00 r. i" ‘ solwcmh was not aged,“ bachgrflum would u Lib.“ n on n this case a - mesmf—wm CHEM14BIi_A__Sl 1 ...
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