Answer_Key_for_PS7

Answer_Key_for_PS7 - Chem 208 Spring 2007 Problem Set 7 Due...

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Unformatted text preview: Chem 208, Spring 2007 Problem Set 7 Due Friday, March 30, 2:00 PM Suggested Additional Problems: ChI7:3,31,33,4l,45,53,59,66,71,73 Name: Lab TA Name: Lab Time/Room: —-——-——-—u_—..w.___' l. A buffer solution contains 0.35M NH3 and 0.50M NH4CI. In this problem, you will usc three different methods to determine the pH of the buffer solution. a) Set up an ICE table for the following base equilibrium in order to calcufate [OH']. Then compute the pH: NH3(aq) + H200) <:> NHflaq) + 0H'(aq) Kb = 1 .3 x 10'5 Ifn) 0.35 050 o C m "X {—5 1—): g 035—): 050+): x . 1g: 0.52:;2?) 19x10}; 0. ex Fr 4mm rec 0.35;2t<<0-S'0 [OH']=I.Z£::IO H .- __ ’F‘f _ 00:5?” : Kym; I [1430+]- :gélf/a’r‘7flyr/0 ’o x _-' 120/0 ’5 P H __ gm Check LZLHO'J- 035. “we = 0.063590 on. b) Alternatively, you can setup an ICE table for the acid equilibrium reaction. in this case, you need to calculate K, for NH4+ using Kb for NH3: NH-Haq) + 320(1) <=> NH3(aq) + 14130+ (at!) I 0-30 0-35 C —-—K 1"9': 4—K g ago—X 0.39,: x —"1’ ~10 4/ = $34 ; l0 : —_ a, W5- 5 YER K — 035M Kl yyrx/o"/° a" 0-5’0-K Guru, KéC 0.3Sx; {yrs/0"“ 0.f0 Chem 208. Spring 2007 Problem Set 7 Due Friday. March 30. 2:00 PM c) Finally. use the Henderson-Hasselbalch equation to calculate the pH ofthc buffer solution. 13”?"er J03“, 40 /0 0.35 H A .0310 £59“) + 7"” d-So : 9.255~0./55 lsz ‘i/OI d) Calculate the pH of the solution that results when 0.15 mol ofgaseous HCl is added to 2.0 L ofthc buffer solution described above. + .—- HC( [5 a sham, acid“ #Cl—f #2104 #50 +5/ 7(- Mswfifa/r'uylian law: {:3 //O'*+ fl/é —>x{/#/Y fA/dO J A M/ 7“ 0'fymofef a"? 11/4), ('5 (anaeréc/ 7 _____-___ — l .u im’z‘m’“ 050%” iiéfiai“ {memos 5"°°’"°‘e‘ 0-15“ Chang}, +0.15 ~05 "CNS M 7 l f/ 5 0.55 - mlcS _ Engont; .. K la LIE.) PH F A (in) r. _ 0-275M (1253+ loam 0.575%, 1‘ 61.25 + “0.320 Chem 208, Spring 2007 Problem Set 7 Due Friday, March 30, 2:00 PM 2. A chemist needs a solution buffered at pH = 5.00. The following acids. and their sodium salts. are available. For each system. calculate the ratio [HA]/[A-] necessary to achieve the desired pH. Which system will produce the best buffer? Explain your reasoning. i. Chloroacetic acid (Ka = 1.35 x 103) ii. Propanoic acid (K.d = 1.3 x 30'5) iii. Benzoic acid (K. = 6.4 x 20'5) iv. Hypochiorous acid (Ka = 3.5 it 10's) l ' .,, Dd E36” Scale S‘olfl PH: PKG [05b [HR] l0 - m K 310 [hi/fl P f2 a ._ Ha] Clflldouace'lTC. 2. 9‘7 Soc-2.5” =2.I3 0.00m; PFOPAAOiC. Lil“? Soc—'7'.ng 0.“ . 0.775 Benaoie Ll l9 Son-V19 = 03; 0. I55 HDPOClqlarm/S 7HL Soo-7HL=—2,qga 233% The late-5‘IL Bdggr l5 Qlo‘l‘Mie-ecfir («J/‘9" [PM]: liar], file when %—%x Alain»: eczwtl fiua/tfllA-P} Lulll lead +0 ‘l’LQ S'mallel‘l' clan/x62 in %§%] (2A?) ‘l’l‘tUS Chanéfil in uFun agfioll'liorx a? and 0f l’JatVL ‘ ' Profmaoic. A Cid /’Progc\noale I: Aeva Chem 208. Spring 2007 Lu Problem Set 7 Due Friday. March 30, 2:00 PM For over 80 years. phenolphthuiein was the active ingredient in Ex Lax. a common over-the-counter laxative. in order to deter students from using laboratory ethanol for inappropriate purposes. such as in punch rtt parties. small quantities of phenolphthalein were sometimes added to laborator}r ethanol supplied to undergraduate labs. a) it" ethanol spiked with phenolplithalein is used in a fruit punch (pl-I 1 5.0), what is the ratio of concentrations [Min] to [In']? is it likely that the presence of phenolphthalein will affect the color ofthc fruit punch? The PK“ 0i: phenolph‘irimiein i3 almi— qg. flu: 051m) ‘H—e “ Henderfon Hurelécht 82"}!- at” f/tolt-Cq'jlufj- PH: Phin—hlojlo DAJ .__.—...—— E H13. Eo—fi3=—€3 tin-J : o {c -: [HIKE i) in lied we} to io‘*3=fioaxm*5 ll uzo.:tn”+np* Far Ho‘s indicu‘vf “It a be Color/:13: ' b) An unscrupulous Chem 208 student decides to test a sample of laboratory ethanol for the presence of phenolphthaiein by adding some ethanol to water and adjusting the pH. In general. il‘90% or more ofan indicator is in the H11] form, the solution will take on the acid color. If90% or more is in the form ln‘, the solution will take on the base (anion) color. What is the minimum p11 required for the student to observe the characteristic pink color in a phenolphthalein solution? To obfe/VQ PM if; Co [or— E l hi] [inJ-tEHlnj i:- Win} 3 O-qogflnfl rim/.3} [in-3 3 0.40Eln'flf690EI-Hnj Odflnflj ,>/ 0.Ci0[H/nj EMF] % 1in oj0;9 Chem 203. Spring 2007 Prohiem Set 7 Due Friday. March 30. 2:00 PM 4. In this problem, the titration curve will be determined for the titration of a solution containing 15.0 mL ot'0.100 M NH; with O. i00 M hydrochloric acid. The Kb for NH3 is 1.8 x 10's. :1) Calcuiate the pH of the solution after addition of the following quantities of hydrochloric acid: i) 0.00 mi; ii) 10.0 ml; iii) 14.0 ml; iii) £10 ml; iv) 16.0 ml; v) 20.0 mi; vi) 35.0 ml Do your calculations on separate sheet(s) and staple them to this probiem set. Plot your points and draw a smooth curve to display your titration curve below. 0 5 10 15 20 25 30 35 mL Titrant added b) Which ofthe following indicators would be suitable for this titration? Refer to Fig. 17-8. Mcthyi vioiet, bromphcnol blue, bromcresol green, phenol red, thymolphthalein, alizarin yellow. m 91 uiuitlérme VDOIAJI CorreyPoficl) 413 FH; 5128/ gromoc rw‘bl aren't Waulol 12? loesrtt) lat/7L [)fomo’o ltdu! lQlJQ and leno/ Wat/[0' Otis—0 waif/C ‘ ...
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