Key Exam 3 - CHE 214 Exam #3. Fa112007l NAME: i I. (10...

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Unformatted text preview: CHE 214 Exam #3. Fa112007l NAME: i I. (10 points) Circle the correct answen’s (There could be MORE THAN 1 correct answer) I. From the titration of a weak acid with a strong base, the pKa can be estimated from the titration curve shown below: 1 —_—_"' L. i, . .. . _ . . | The pKa of the weak acid titrated above is approximately: a. pKa:11.5 c. pKa=8.0 \ b. pKa:10.0 pKa=4.2 2. Refer to Table 12-4 (last page): Select the appropriate indicaton’s for the titration in question #1 and indicate the expected color change that you will observe. a. Erythrosine. from to b. Bromcresol green, from ,____ [0 ——__ [ C9 Bromthymol blue. from L13 to B I fig . __—__ t0 _——__ d. Nitramine, from DJ Which of the following paira’s constitute a buffer: a. 0.100 M HBr and 0.100 M KOH b. 0.100 M HCl and 0.100 M NaCl t @0. 100 M NaHCO3 and 0.100 M Na2C03 d. 0.100 M HNOJ and 0.100 M NaNO3 4. Which of the following statements isfare TRUE about buffers: a. Buffers will always resist change in pH even if you add large amounts of highly concentrated strong acid or strong base 3 @The pH of the buffer will change with drastic change in temperature The buffering capacity is highest when the solution pH is equal to the pKa of the acid @ The pH of a buffer is relatively independent of the buffer concentration 5. Consider the amino acid cysteine with 3 ionizable protons and their pKa values: + + + 1TH} pKlzlfn hill-l3 pK2=8.36 {fl-I3 ' pK3=10.?7 THE CHCHZSH _.—_'- CHCHZSH ‘——'—""‘ CHCHZS --——""‘ CHCHES (l; K|=0.0195 (I: K2:4_37x10 -9 (I: KELjr'Oxm —11 | I, x I; \ - I! \ — _ ch 0 OH 0 0 0 O o’ 0' (A) (B) (C) (D) Which of the following statementfs is/are CORRECT? (9 At pH: 5.04 = 11’2 (pK1 + pK2), the concentration ofA is equal to the concentration ofC Lt At pH: 5.04 2 11’2 (pK| + pKz), the major fonn of cystein in solution is B @ At pH 10.7?, there are equal concentrations of C and D ® At pH 12.00 the the major form of cystein in solution is D 11. (6 pts) The control of pH in experimental chemistry is essential in all sorts ol‘chemical and instrumental applications. Ofien the success in these applications is determined by how carefully the pH is controlled and maintained. Thus, buffered solutions are of great significance. Suppose that you are conducting an experiment which requires the pH of your solutions to be maintained at pH 71.0, which of the following buffers will give you the best buffering capacity? ENCIRCLE YOUR ANSWER, and explain briefly why you chose that buffer. [(3 _& Formula mass a) Citric acid 141x 10'4 3.1; 192.124 6'3—(N—Morpho1ino)-2-hydroxypropanesu1fonic acid 1.1? x 10‘7 6. Q3 225.264\3 c) N-Tris(hydroxymethy1)methylglycine 7.0 x [0'9 ‘3, 1:; 179.171 1kg BugaA‘aoX Carnal"? 1x5 “Wklmwx 901* PH claifi ’l’D Pica [I]. (15 points) How many grams of NaOH (MW=40.0 gfmol) must be added to 2.00 L of 0.250 M solution of oxoacetic acid (HCOCOZH, MW=74.0 gimol) to produce a buffer of pH 3.85? pKa for oxoacetic acid = 3.46. HCoCOH—k “(4i HCOCoo' Mr Om) R; A” (.7 0300 Mes. MW? —9 £2“) L) CD.Z§0 M) Z: O 500 baby“ H’A “AMI/X Pfifir”“—--—"“ 1‘2 ZOnQQey‘ Q Eek we W Jr OH" __—‘> A" 4— HM) 050w X -——~ ‘ (0 960-0 -———~ >< A'l H :- \C + \O V T? 0‘ 3 [M3 4 3d 3‘39 I 3% Ar lu-b C10 “f3 («ng’WQ X l“ 03:92:}— chov DH” “b70170 fl: answer M Jr : (ca-352.} >(qoo ?I W : Z a Mam ) Wukmfi“ ‘PL' 3‘2?“ IV. (24 points) Consider the amino acid Histidine, a triprotic acid, with the dissociation reactions and pKa shown below: C02“ co; — + = H3His2+ co; — NH2 pK3 = 9 28 His— IVA. (6 points} Provide the following information __ r—G —. o new K32: 1.0} ND K33: S. 25 we ’ IVB. (2 points) Encircle and label the form of histidine at its isolectric point. IVC. (3 points) What is the isoelectric pH of histidine‘.’ \ __ ?H: 3/ (g'fi?+ 5M3} Shiv-(3 ‘2?“ ?H'; :h (02 (PU- OHQUB P v’ IVD. (3 points) At the isoelectric pH of histidine, which species have the same concentration? [Use the symbols HJHis”, HzHis”, HHis, His" ] ’: HZHEISRt—l : [Hisuj IVE. (10 points) A 0.250 M solution of the monosodium salt of His- (fully deprotonated form of histidine) was made. What is the pH of this solution? I ,3 '— A <: WA rt 0 “ :‘ZSOH‘X x X V. (25 points) A 50.00-mL of 0.750 M of acetic acid(weak acid, pKa: 4.?5?) was titrated with 0.500 M KOH. Titration reaction: CH3COOH + OH' jl-ECOO' + H20 A. Fill in the blank: M F'E-————_____ a) mmoles of acetic acid to be titrated: E ‘ s E h % 3:)” g- n‘ I"1: b) volume (mL) needed to reach equivalence point: t 031 5 Malis ‘B. What is the pH of the solution at equivalence point? Pd“ \olr. all CHCOOH has 5‘2?»- COWE’Vl—efl +0 C(4me- 0.0 l UHMOD’ 3: 3% M” : 0.300 Heb/ CDJDSOOL. 4-0-0} L‘ L Wk flu t mower A" + H10 *3- W + 0”” 0.3m—x K )C kb: @3323; W3 99%» 3“ w? Sch—flaw : C__2x)(x L6 565%»: K<s<< Roe (em—x) X: “5‘ “’5 =1 Low} Nb 3 \ Qt (Wt/39” C. What is the final pH of the solution after adding ?5.15 mL KOH? Exam-5 our“ ~: O-KS‘mL VI. (20 points) Acetylsalieylic acid (a.k.a. aspirin) is much more thanjust a pain killer; it is said to ward off heart disease and prevent thrombosis, and this drug has been mooted as a possible treatment of cancer, cataracts. and senile dementia. O 0 || || _ C—OH Ka:3.lo x I04 C—O _._._I-.. + O—fi—CH3 "-— O—fi—CH3 + H O 0 Acetylsalicylic acid (aspirin) dissociated form oi'acetylsalicylic acid HA A' A. What is the pH ofa solution containing 0.100 M of aspirin, HA? HA V—E Adi .p Hii 0.100%»: K x _ e a : (Xxx) MSe : I I C) O Inn—- ‘ \(a 3 79 (ll-{U0 # 7r : irlaaaipe‘in J20 (lump X: 0.003%“, 2.: 5443 PM “*5 22c B. What is the fraction of dissociation (1A- and the fraction of association (In A of a 0.100 M solution ofthe drug. ...
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This note was uploaded on 04/19/2008 for the course CHE 214 taught by Professor Aga during the Spring '08 term at SUNY Buffalo.

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Key Exam 3 - CHE 214 Exam #3. Fa112007l NAME: i I. (10...

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