Lab 9 Potentiometric Determination of an Equilibrium Constant

Lab 9 Potentiometric Determination of an Equilibrium Constant

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Unformatted text preview: Name Section 006 EXPERIMENT 9 POT ENTIOMET RIC DETERMINATION OF AN EQUILIBRIUM CONSTANT PRE-LABORATORY QUESTIONS The following preparatory questions should be answered before coming to lab. They are intended to introduce you to several ideas that are important to aspects of the experiment. You must turn in your work to your instructor before you will be allowed to begin the experiment. Potassium acid phthalate, KHP (KHCgH4O4), is a primary standard reagent used to determine exactly the concentration of a solution of base, such as NaOH, Whose approximate concentration is known. KHP is a weak acid, and the equation for the neutralization of KHP by NaOH is (Z KHP Potassium acid Phthalate 1. Samantha E. Muir dissolved 0.31 grams of KHP in 25 .0 mL of water. She found out that it required 15.5 mL of NaOH to reach the endpoint of the titration. a. How many moles of the acid, KHP, were originally present before the addition of the base? . 01 iii/cert g ’ 3 f // . 4x233”? : fifixfl we .Zre. » i ; Iv" b. Calculate the concentration of the NaOH solution? I; m gag, e s 3 f I : N _;’ f r‘ /‘ é Z21“) g, x /«S N6» #9" ,rfl/é a. Cheml 5 l SfiAcademic Year 2010/201 1 c. What is the pH of the solution at the endpoint? 3C * We; KMPMJV I 524' {if H 1165 1 1231;; 7’ [512159—77 415315.150 0 r“ / C /” 565?};1 F . '"b (raw; , K34 ‘1” j {3,3 .. y f 90 C x M N?” 1%“: a, . . 5 7,: {N “’11; AK ,1 V a I ‘ i “ff; 1‘ [AM i) I 5 .. .,.— " 3.134" ' » -14: -« We!” 1 r“ i’ 5’ 3 gig: £7 a! b 4 /,§ h.» I. _. 2 ‘1 6/,02A A} .. d. Given the following table of indicators, pH ranges and colors, which would be the best indicator for detecting the endpoint of the titration? 5‘? , '11 '/ ,n" :v E‘s _ ' (5‘ F“ “M {Q m ‘57" 9/1445}: (“52‘ f" ' I 4 - 4": “av I Indicator Meth lorange Bromocresol reen Meth lred Bromocresol purple Phenol red Phenolphthalein Thymol blue Diazo Violet — 3.1 iRed/orange 4.4 — l4 iYellow 4.0 iYellow 5.6 - 14 iBlue . iRed 6.2 — 14 iYellow 5.2 EYellow 6.8 — 14 gPurple - 6.4 EYellow 8.0 — l4 §Red - 8.0 Colorless lO - l4 Red — 80 iYellow 96 — '14 iBlue 0 — 10.1 iYellow 12.0 — 14 iViolet Cheml 5 l SfiAcademic Year 2010/2011 972 Section 0 09' 2. In a second titration with the same solution of NaOH as used in Question #1, Samantha used 0.29 g of KHP. Calculate the volume of the NaOH solution needed to neutralize i this sample ofKHP. ‘2qu Y pm: if [,E £7, 2 y (Mrs M H: U; fret ~~> 53 I. V‘ , t w, .t..~u_.,»——" \A \ j if.“ » \Lt i" “L Mel, NQC/fly “w figéé’ZL C " . .2; c J 3. Write the neutralization equation which describes the reaction between the weak monoprotic acid such as hydrofluoric acid, HF (Ka for the HP is 6.4 X 104) and a strong base such as NaOH. H/Z ._.- v at a 1 [5:1 O "rag/:2 » MC 7L AZ? L11 Lag} t 4. Mr. B. S. Browning was given a 50.0 mL sample of 0.110 M solution of HF. Using a buret, he transferred 25.0 mL of the weak acid into a 250 mL Erlenmeyer flask. He then titrated the acid with a standardized solution of 0.0987 M NaOH. a) How many moles of the weak acid were added to the Erlenmeyer flask? fr '7‘ a:ii‘:3 é:f?§,l‘ ‘W @52ny uwz'fi’wc’f b) How many moles of NaOH are required to neutralize (reach the equivalence point) the sample of weak acid? 5 (\x ,. 'y 3 ».« f f‘ 'v. 1 00 -‘ »‘ Wad 049’ 74‘s: if :i fake «? c) How many milliliters of the NaOH are required to neutralize the sample of weak acid? 7 “I 3’: ft: ‘ 3 ‘m 3.. LE 2: f ,3 _ j I- D a q ,7 M A i i V“ w (“#5 :10 mam-k Chem15 lS—Academic Year 2010/2011 d) How many moles of NaOH have been added at one half of the volume in part ‘c’ ,\ (volume at the half equivalence point)? ‘ u 2 van“: ‘ - = Wm; a .1715.— X33~WW7> : /,zs>x/o 3W“)! L- H / . i Zésmé. 6) How many moles of the weak acid have reacted at the half equivalence point? 2 a /, 2% we“ ‘9 méaé f) Calculate the pH of the solution at the half equivalence point. *d V“ 5‘“? - 55%;. J t -' J ,y' , K01 ' wfiww. ; :1“ fl'w {a i live! .f l , M 4:5 /\/~* élV/fi/O L’ cilfib j film 3‘ A; w F at“ W * ~ ’5‘”; g) Explain how the pH at the half equivalence point is related to Ka for the weak acid. ff} a V5.1? _ . .4 ‘5‘ “fl 1 r a ” 4,’ Q I; \Q K W J a {L - z i é £5 w“ ChemlS 15—Academic Year 2010/2011 9 4 m a f Section Q5? RESULTS PART ONE : STANDARDIZATION OF AN NaOH SOLUTION WITH KHP Trial #1 Trial #2 Trial #3 Mass ofKHP é g .63 a? I éo Z i 2 i Initial volume of NaOH (j ‘ I l “‘6 ., 1m ‘ ,1 EH a, _, Z W. L Final volume of NaOH I _ g l {i {bit-in Iq‘gml L fN OH dd d , r— 3, , Molarity of NaOH ,O‘WM ,§?#M 400V”? Sample calculation of NaOH molarity for Trial #1: 0 7703aé Calculation of average molarity of the NaOH solution: C05"? 430?? 1r,f00\m " 21’ .in m a”? WWcamw remark“) ChemlSlsfiAcademic Year 2010/2011 PART TWO : POTENTIOMETRIC TITRATION OF AN UNKNOWN WEAK ACID #1 Unknown Number: Tiration # 1 Titration # 2 Initial volume of unknown ; f acid (buret readingL 5:): a W /fl 49M L Final volume of unknown (- ._ a A . ,- I Q m, acid (buret reading) i'érgvfi Z 5/“ volume (mL) of unknown _ Ia—cid ' i if} git? Z g I 0 W1 / J Initial volume of NaOH_[ / ;/ rat w (buret reading) 0 ’ 0 o W L ” i 6 Buret reading of NaOH at ‘ [a r“ , . . W L I x z equivalencemnnt _J é I 2» ! ’" volume (mL) of NaOH at Z W I; T A? E: If eguivalence p_oint " ‘ V V e“ a Molarity of unknown acid : , 0% (:2 Sample calculation of molarity of unknown acid for trial #1. Calculation of average molarity of unknown acid: Cheml 5 l SfiAcadernic Year 2010/2011 9—10 H mL NaOH .. 2 n n A, 0 y .l . t W. J m Al _. t 1M: I? Q: a; la II .1 29 y A.» (22“ f i, L 2 a. M Q ? an; T Hui/MA,/n3u,m9a%mn H1§mwz,o? 5V3; w,z.,:- MW/v , Q V r , ,, . a I 1 . rm. w/u AV 1H ‘1 . Z (V. .. 2,2,, “#Hiufwwééééonaw fA; . fl;:,, (NJ 7 n O .U C e S mL NaOH 6 '3 - , {a Q a 7 O 3 VL— 2 2 Date mL NaOH E H *3 Titration 1 3 ) Q, :n I L/ {a a: J . an ‘ r I . v 2 a 7 d d n a m k C .w h C n e b m e d m S e m m a W W a m p .E m e m .m W e H A P U 0 N G I S TA a. a 9~11 Chem15157—Academjc Year 2010/201 1 Cheml 5 15—Academic Year 2010/2011 2 Chem] 5 ISfiAcademic Year 2010/20] 1 Titration #1 Titration # 2 Volume of NaOH at equivalence point L m L T 4 A 0 W C Volume of NaOH at 1/2 equivalence point 22 I l n: L % VD Wl' I; pKa (from plot) a, 6 q, { Ka Zizz’wzo'g 2245200"; Sample calculation of Ka from pKa for titration #1: WKQ I; WAC??? i [713 >“[e—7 ’7151/09 <53 / /6"”<:}(¢q Calculation of average Ka: ‘é‘ g/l’étffa L Z Cheml 5 l 57—Acaderm'c Year 2010/201 l 9~14 Section & 0; POST LABORATORY QUESTIONS 1) The following data was collected when a 25.0 mL sample of an unknown monoprotic acid was titrated with a 0.100 M NaOH solution. Volume of NaOH EH Volume of NaOH _, EH 0 1.2 24.5 4.1 L 5.0 1.3 25.0 5.1 8.0 1.36 26.0 5.5 13.0 1.5 28.0 5.9 20.0 1.9 30.0 6.0 22.0 2.2 32.0 23.0 3.0 34.0 . 24.0 3.8 36.0 6.25 Make a plot of the titration curve and then determine Ka for the unknown acid. IZO mg. :4 A... m1 of 0.1 M NaOH added ChemlSlS—v—Academic Year 2010/2011 5 2. Sketch the following pH curves in the space below. (Put all three curves on the same graph, but use different colored ink (pencil) to distinguish the solutions.) "~ when a 0.100 M. solution of a strong monoprotic base is added to a sample " \ of a QAIOO M solution of strong monoprotic acid. / @. when a 0.0100 M solution of a strong monoprotic base is added to a 20.0 mL sample fr/of a 0.0100 M solution of strong monoprotic acid. _ @when a 0.00100 M solution of a strong monoprotic base is added to a 20.0 mL sample of a 0.00100 M solution of strong monoprotic acid. 14 l ‘ . rd 1 y 1 n. . 12 'i l r i 1 i 2 1 e H 10 l f ; 1 i It. 1 . 1 pH 8 ’1 l 1 A“,— i 6 i ii i :11 i g 4 i 2 1 3 z ‘ t t; :17 ~ ’ _, a e s I“, V 1 ’ i i {5 J I ’ 0 5.0 10.0 15.0 20.0 25.0 Volume of base added . ~55 Q. (,00'76L)<o/Im\) : 1159......“01:@H.] : ’oléé’m .o‘méz. Z i 5.5:H” fl / 1+ a f w Chem1515——Academic Year 2010/2011 9—1 6 ...
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Lab 9 Potentiometric Determination of an Equilibrium Constant

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