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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
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(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,
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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
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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
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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 .
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Ex Ianation O Sentence : . r ‘ .9
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. . .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
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-...————--.—-——.——-—-—--——- 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
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- Fall '13
- pH, titration graph, law calibration graph