105bf07_sample_e2

105bf07_sample_e2 - Sambafll Fame 200 Chemistry 105bL...

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Unformatted text preview: Sambafll Fame} 200? Chemistry 105bL First Letter of PLEASE PRINT YOUR NAME IN BLOCK LETTERS 11:00 am. Section Last Name Fall 2004 Name: EXAM #2 i g if > October 5, 2004 Last 4 Digits SID #: Professor Arieh Warshel Lab T.A.'s Name: Lab Day and Time: Please Sign Below: I certify that I have observed all the rules of Academic Integrity while taking this examination. INSTRUCTIONS 2 1. There are 5 problems on 7 pages in this exam. Please check to see that you have a complete exam. 2. Please show all of your work to receive full credit. 3. You must use ink. Do not use a pencil. Useful Physical Constants and Equations: Avogadro's number N = 6.022 x 1023 mol'1 Universal gas constant R = 0.08206 L atm K" mol'l = 8.314 I K" mol'1 Faraday's constant F = 96,485 C mol" —b Jbz Ifax2+bx+c=0 thenx=-—i-2#- PV = nRT 'W = (lg NA] = 4“ + ln[A]o AS=k1nW KP/Kc=(RT)A“ AG=-HF§ t1/2=O'6T93 tl/2=k[+A]ol AG: AG" + RTan AG = AH — TAS ln(k) = EYE) +ln[A] E=q+w AG” = —RT1nKeq —%.9=—i1.‘5—+AS E = ho = mc2 Values of K“ for Some Common MonoRrotic Acids W Hydrogen sulfate ion HSOF 1.2 x 10‘2 Chlorous acid HClOz 1.2 x 10‘2 Monochloroacetic acid HC2H2C102 1.35 x 10—3 Hydrofluoric acid . HZF 7.2 x 10-4 Nitrous acid HNOZ 4.0 x 10‘4 Acetic acid HC2H302 1.8 x 10‘5 H drated aluminum III ion — Hipochlorous acid ( ) gfigzokl“ :2: 13‘: Hydrocyanic acid HCN 6.2 x 10—10 Ammonium ion NH: 5.6 x [0'10 thl H0C6H5 ' 1.6 x 10-10 [A] ti/z = TkO’ 1. (20 points) 4‘04: 6464/ Consider the following equilibrium: \ x \ 2mx<g):2m(g)+&(1)+heat 43/” =0 X2, ’5 “V /’Z'“’“(' (a) Decreasing the pressure in a constant volume will cause (i) the reaction to produce more 1-sz (ii) the reaction to produce more H2 and X1 (iii the reaction to produce more H2 but no more X1 no shift in equilibrium v X1 to dissociate (b) Addition of X1 to a system described by the above equilibrium @ Wlll cause [1-12] to decrease M05) W #6 (4/1” 6e, (ii) will cause [X] to decrease ( “(flu/{'- Marc‘s; Since, (iii) will cause [1-12X] to decrease (iv) will have no effect “(Yb WM 44’4“! (v) cannot possibly by carried out (c) Increasing the temperature will cause a fl 2 414;? P ”44 a‘f @ the reaction to occur to produce more H2X ii) the reaction to occur to produce more H2 and X2 (iii) the reaction to occur to produce more 1-12 but no more X1 (iv) no reaction to occur (v) an explosion (d) The equilibrium constant of the reaction nA —) mB increases with increasing temperature. Is the reaction exothermic or endothermic? Explain your considerations. M ’54“ 40197, M d" reacfiwt =§Mfler~fa flank/fer 3 if mm react“! 1‘s 4/124 Me 524141.361”: 7‘0 flier/W, (e) What will be the effect of a catalyst on the equilibrium amount of H2? 2. (20 points) The acid dissociation constant of a weak acid HA is 5.0 x 10". (a) Calculate the pH and pOH of a solution which contains 0.020 M of HA (clearly ’0 (+5 state any assumption(s) you make). HA: H++ A " 0.020—)6 X X 2. ,_ . K = K = .ox '9~ X ‘2“ fl Xéw'ozo a. 0.020-x 5. ’0 ~ 0,020 2 7 “E : ~ " fa K v [OX/o 5- ,———:‘:: x/afi: 027. X = 3.2)0; = 6",]: 0’7 £44 KnurZ/ pH= 41/? M n 453%" ’57" - 4. 7 ’0”: 7.17 7—5-37— (b) What is the pH obtained by dissolving 10'10 moles of HCl in 1.0 liter of water? ‘ -o -—7 -— Sr“ ,o’H +105 3/051 H* ' do ,H': 7,0 - (c) What is the K8 for the dissociation of BH+ to H‘ and B if Kb = 10.5? 5‘ 7L: Kw " kk/C; lb /< / 0X/ 5” ~ (a: L = _'___.——— : LOX/o /. OXIo': Kb 4 3. (20 points) Phthalic acid (HzC3H4O4, abbreviated HzP) is a diprotic acid whose ionization in water at 25°C takes place in two steps: H2P :H‘ + I-IP‘ K;,,=1.25x10‘3 I-IP‘;H*+P’ Ka2=3.10x10*5 (a) Calculate the P= concentration obtained after dissolving 0.040 mol of HZP in 1.0 liter of 8"”: water. -—3 X '— .. [.ZS‘K/o = log, 0.007] _ 7. 3- K.‘ = Luna = L 2; __ ' o'oV'X a ay- 2_ -$' - _ X ‘$.0X/:3 ‘HF— : P2 ‘l’ Hi3 X = 7”" (55300)“? 7 WW )“7 -3 —3 -6 gmzmg Z)- ; 7 ”If; X/uz : I77 Kaz=3l°h° ;( M.” —3 .. 2’ 4-. Ono -6 ‘ o . f 40 3’ = 3J0 no =Ka . =>4§r~dz$nzwdif , VP 3 > W O """""""" (b) Calculate {he concentration of [P‘]:flwe dissolve 0.100 mol HZP in 1.0 liter of solution 4 P+S whose pH is kept at pH = 9.00 by a bufier. Am Mum MM; «am mama W W P2“ M [fz-Jz fl./00fl HPP+ ZOH' ”'9 P9" +2+{,,oy 3 J} (c) Write a charge balance equation for the solution in part (a). [W7 = [HP']+ 1ZP”]+ [or] (d) Write a mass balance equation for the solution in part (a). 3 ft 0.0th = [MN [HF'] + [F27 5 . 4. (20 points) 0.500 moles ofNaF were added to 1.0 liter of0.500 MI-IF (K. = 7.2 x 10“). (a) Evaluate the pH of this solution. W3 (b) Is this pH larger or smaller than the pH before the addition of NaF? Please explain. 6F+S HIM 0“ MP} Wt hm HFJS‘H-f' +F~ Add"? UaFM4/%WMWF‘I 4 ”Mat. @1427va d» #flozfg ’49 [Hf] \l/ W [Hf Wflam S‘s/FHAWV effingfiwm 5. (20 points) (a) The molecules XH and YH below are weak acids with regard to dissociation of the Cm-H bond. However, XH is a stronger acid than YH. Please give the reason for this, assuming [Del—5 that the Cm-H bond strength 15 equal in both cases. <3 *l H Hr ,“ /C\ ¢C\c1’ ,C‘C{ H C /\ ’ H |\-1 H H H H Hf HX HY #1L W C? _. laugh ”(my #54:} c'xngQ CW, fo Sh "5 H /? /C\ y :95 X We H H l-f V MW 40 mt, with” (D H "2‘ h (b) The following acids are arranged according to their relative strength (the strongest first). HC104> sto4> H3O+ > HF > NH: > HCN (2 9+5 @060!) . Jab/e « Circle True or False for each statement below: . Sn Kc f, ”F "I." 6‘4 Mama! In ”Ii/3. C9 (i) HF will behave as a strong acid in NIL True ”71'; acid 5W4“: 40'” A! Mr - (ii) The acid strength of I-IZSO4 will be larger in "‘ 909M ' benzene (non-polar solvent) than in water. rue @ T /? Ka A". ”F [‘5 Shall” 7%“ 4M %!6( (iii) HCl is a stronger acid than HF, therefore the dissociation constant of HP is larger than that of HCl. True 2 w, is m “73.4. m. a; My. (iv) H3015 the conjugate base of 4 £5 ’ H1013 4.“ True (v) HCl is an acid by the Lewis definition. J ‘9 Z! True x parka wit/ti“ “f ”59*- dec‘fvavy i:- or: ac: ”a [‘5 M a6!” V / ”I? 67 ML grandad-Low? Mmzm. ...
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