CHEM 108 (Fall-2003) Exam Final (100 pts)

CHEM 108 (Fall-2003) Exam Final (100 pts) Name: -------------------------------------------------------------------------------, SSN -------------------------------- LAST NAME, First (Circle the alphabet segment of your LAST NAME): A-C D-H I-M N-R S-Z Please answer the following questions: Part I: Multiple Choices (45 pts: 15 @ 3 pts each). Circle the ONE best answer: 1. With what methods are the transuranium elements formed? a) neutron bombardment of uranium-238. b) the transuranium elements occur naturally. c) positron emission from phosphorus-30. d) α-particles bombardment of uranium-235. 2. Solid gold(i), AuCl (K sp = 2.0 x 10-13 ) is dissolved when excess, CN - is added to give a water soluble complex ion, Au(CN) 2 -. If K f for Au(CN) 2 - from Au + and CN - is 2.0 x 10 38. What is the value of K for the following reaction: AuCl (s) + 2 CN - (aq) Au(CN) 2 - (aq) + Cl - (aq) a) 4.0 x 10 25 b) 8.0 x 10 12 c) 2.0 x 10 38 d) can t be calculated from the given data. 3. Which of the following statements is FALSE: a) Reactions with a negative H rxn and a positive S rxn are product-favored at all temperatures. b) The entropy of a pure, perfect crystal is zero at 0 K. c) A sample of pure I 2 vapor has higher entropy than pure solid I 2 (both at room temperature). d) Reactions with positive H rxn and a positive S rxn can never be product-favored. 4. For the reaction A B, the activation energy, Ea = 125 kj/mol and the heat of reaction, H = -50 kj/mol. What is the E a for the reverse reaction in kj? a) -75 b) 175 c) 75 d) -175 5. Both sodium nitrite, NaNO 2, and sodium benzoate, NaC 7 H 5 O 2 are used for food preservatives. If solutions of these two salts have the same molarity, which solution will have the higher ph? K a (HNO 2 ) = 7.2 x 10-4 & Ka (HC 7 H 5 O 2 ) = 6.3 x 10-5. a) sodium nitrite, NaNO 2 b) sodium benzoate, NaC 7 H 5 O 2 c) both will have the same ph value d) can not decide from the given information 6. For second order reactions the slope of the reaction of a plot of 1/[A] vs. time is a) k b) -k c) k/[a] 0 d) k/t 1

7. What is the standard cell potential, Eº cell for the reaction: Cl 2 (g) + Fe (s) 2 Cl - (aq) + Fe 2+ (aq) Given that: Cl 2 (g) 2 Cl - (aq) Eº = + 1.358 V and Fe 2+ + 2e Fe (s) Eº = - 0.440 V a) +1.798 b) -1.798 c) +0.918 d) -0.918 8. What would be the osmotic pressure, in atm produced at 25.0 ºC by a 0.010 M NaCl solution? a) 0.489 b) 0.245 c) 24.8 d) 4.11 x 10-2 9. Which of the following statements about the kinetics of reactions is not correct? a) A catalyst lowers the activation energy of the reaction. b) Reaction rates increase in the presence of a catalyst because collision rates are increased. c) Collisions of insufficient energy do not result in a chemical reaction. d) None of the above is incorrect 10. The ph of a 0.30 M solution of HCN is 5.20. Calculate the K a value for HCN. a) 1.3 x 10-10 b) 6.3 x 10-6 c) 4.8 x 10-2 d) 2.1 x 10-5 11. Initial data were obtained for the following reaction: 2 A (g) + B (g) C (g) + D (g) Expt No. Initial [A], mol/l Initial [B], mol/l Initial rate, mol/l.s 1 0.10 0.15 0.45 2 0.10 0.30 1.8 3 0.20 0.15 0.90 What is the rate law for the reaction? a) Rate = k [A] [B] 2 b) Rate = k[a] 2 [B] c) Rate = k[a] [B] d) Rate = k[a] 12. Examine the titration curve shown. Which of the following titrations could it represent? a) HCl by NaOH b) HCl by aqueous NH 3 c) NaOH by HCl d) Aqueous NH 3 by HCl 2

13. Consider 0.10 M solution of the following substances. Which would have the lowest ph? a) KCl b) NaC 2 H 3 O 2 (sodium acetate) c) NH 4 Cl d) KOH 14. Which change will increase the quantity of product in the following reaction? 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) Hº = -198 kj a) increase temperature b) add a catalyst c) remove some SO 2 d) increase pressure 15. Some PCl 5 (g) with a partial pressure of 0.500 atm is heated in a closed container and the following equilibrium is obtained: PCl 5 (g) PCl 3 (g) + Cl 2 (g) The equilibrium pressure of Cl 2 is 0.254 atm. What is K p for this reaction at this temperature? a) 0.254 b) 3.93 c) 0.262 d) no answer was given Part II: (20 pts) 1. (10 pts: 5 @ 2 pts) Complete the following nuclear reactions, identify X in each case and classify each reaction (alph-emission, beta-emission, positron-emission or electron-k-capture, bombardment reaction): X Type of Reaction a) 238 U + 1 0n X + -1 0 e --------- --------------------- b) 238 U X + 4 He --------- --------------------- c) X d) 82 Rb + X 207 Bi + +1 0 e --------- --------------------- 82 Kr --------- --------------------- a) 14 C isotope is the basis for radiocarbon dating technique. Write an equation to show it can be prepared by transmutation of atmospheric nitrogen ( 14 N ). ------------------------------------------------------------------------------------------------------------------- 3

2. (10 pts: 5 @ 2 pts) Complete the following sentences and/or the required expression for the reactions: a) Write the solubility product expression for Ag 2 CrO 4 : K sp = --------------------------------------- b) For the reaction: 2 HgO (s) 2 Hg (l) + O 2 (g), write an expression for the equilibrium constant (K p ): Kp = ------------------------------------------------ c) The conditions that would apply to a reaction that is spontaneous at room temperature: Gº = ------------------------ and K --------------- d) The greatest gas solubility in water is predicted under the conditions of -------------------- temperature and -------------------- pressure. e) Natural radioactivity is defined as the -------------------------- decay of unstable nuclei with evolution of energy. Part III (35 pts: 5 @ 7 pts) Show all work for full credit. Please express all answers with proper units and correct number of significant figures. Please put your answers in the provided boxes. 1. USP ethanol is an aqueous solution containing 95.0% ethanol by volume. At 20º C pure ethanol has a density of 0.789 g/ml and the USP ethanol, 0.813 g/ml. How many grams of water are present in 100.0 ml of USP ethanol? 6.3 g H 2 O 4

2. Will a precipitate form when 150. ml of 1.5 x 10-3 M Pb(NO 3 ) 2 is mixed with 100. ml of 0.025 M NaCl? (K sp for PbCl 2 = 1.6 x 10-5 ) Yes, because: OR No, because: Qsp (9x 10-8 ) < Ksp 3. Consider the following data and calculate Gº rxn, in kj at 155 ºC for the following reaction: 2 O 3 (g) 3 O 2 (g) Hº f (kj/mol) 142.7 0 Sº ( J/mol.K) 238.8 205.0 Gº rxn = -344.2 kj 5

4. A 0.750 g sample of an unknown substance is dissolved in 20.0 g of benzene; the freezing point of the solution is 4.53º C. Calculate the molar mass of the substance. The normal freezing point for benzene is 5.53º C and K f is 5.12 º C/m. Molar mass = 192 g/mol 5. a) The following reaction Zn (s) + 2 Ag + (aq) Zn 2+ (aq) + 2 Ag (s) has E cell = 1.562 V, calculate the equilibrium constant for the reaction. b) What mass, in grams, of copper will be deposited from a solution of Cu 2+ by a current of 2.50 A in 2.00 h? 5.93 g 6

6. (Bonus question: 8 pts) In the titration of 20.0 ml of 0.200 M HOBr (K a = 2.5 x 10-9 ) with 0.100 M NaOH, what is the ph at the equivalence point? ph = 10.71 7

PREFERENCE SHEET FOR CHEM 108 Exam Final Fall 2003 You will have 140 minutes to complete this exam. The exam has 7 pages plus Periodic Table and Reference page. When you are told to do so, tear off the Periodic Table cover sheet and use as required during exam. Useful Information & Equations: Gas Constant, R = 8.3145 J/K.mol = 8.314 x 10-3 kj/mol.k = 0.0821 L.atm/mol.K K = 273 + C 1 atm = 760 torr = 760 mmhg Standard temperature = 298.15 K Faraday`s constant, F = 96,500 C/mol = 96,500 J/K.mol 1 V = 1 J.C -1 d (density) = mass (m)/volume (V) PV = nrt π = MRT T = i. Km. P solv = χ solv.p o solv. Rate = k[a] ln [A] o = kt ln[a] = - kt + ln[a] o t 1/2 = ln 2/k = 0.693/k [A] Rate = k[a] 2 1/[A] - 1/[A] o = kt t 1/2 = 1/k[A] o Rate = k [A] = - kt + [A] o t 1/2 = [A] o /2k k = Ae -E a/rt = (-Ea/R)(1/T) + ln A ln k Nernst Equations: G rxn = G rxn + RTlnK & E cell = E cell - (0.0257 V/n) ln Q (at 25 C) G rxn = - nfe = - RTlnK E = (0.0257 V/n) ln K (at 25 C) Charge (coulombs, C) = current (A) x time (s) 8

9 1 1 H 1.01 Periodic Table of the Elements 2 He 4.00 2 3 Li 6.94 4 Be 9.01 5 B 10.81 6 C 12.01 7 N 14.01 8 O 16.00 9 F 19.00 10 Ne 20.18 3 11 Na 22.99 12 Mg 24.30 13 Al 26.98 14 Si 28.08 15 P 30.97 16 S 32.06 17 Cl 35.45 18 Ar 39.95 4 19 K 39.1 20 Ca 40.08 21 Sc 44.96 22 Ti 47.88 23 V 50.94 24 Cr 52.00 25 Mn 54.94 26 Fe 55.85 27 Co 58.93 28 Ni 58.69 29 Cu 63.55 30 Zn 65.38 31 Ga 69.72 32 Ge 72.59 33 As 74.92 34 Se 78.96 35 Br 79.90 36 Kr 83.80 5 37 Rb 85.47 38 Sr 87.62 39 Y 88.91 40 Zr 91.22 41 Nb 92.91 42 Mo 95.94 43 Tc (98) 44 Ru 101.1 45 Rh 102.9 46 Pd 106.4 47 Ag 107.9 48 Cd 112.4 49 In 114.8 50 Sn 118.7 51 Sb 121.8 52 Te 127.6 53 I 126.9 54 Xe 131.1 6 55 Cs 132.9 56 Ba 137.3 57 La 138.9 72 Hf 178.5 73 Ta 181.0 74 W 183.8 75 Re 186.2 76 Os 190.2 77 Ir 192.2 78 Pt 195.1 79 Au 197.0 80 Hg 200.6 81 Tl 204.4 82 Pb 207.2 83 Bi 209.0 84 Po (209) 85 At (210) 86 Rn (222) 7 87 Fr (223) 88 Ra 226.0 89 Ac 227.0 104 Rf (261) 105 Db (262) 106 Sg (263) 107 Bh (262) 108 Hs (265) 109 Mt (266) 110 Uun (269) 111 Uuu (272) 112 Uub (277) 58 Ce 140.1 59 Pr 140.9 60 Nd 144.2 61 Pm (145) 62 Sm 150.4 63 Eu 152.0 64 Gd 157.2 65 Tb 158.9 66 Dy 162.5 67 Ho 164.9 68 Er 167.3 69 Tm 168.9 70 Yb 173.0 71 Lu 175.0 90 Th 232.0 91 Pa 231.0 92 U 238.0 93 Np 237.0 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260)