Chem 128, Final Exam May 11, 2003

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1 I. (70 points) This part of the final corresponds to Exam I. It covers the material in Chapters 10, 11, and 12. A. (10 points) Answer the questions below, using LT (for is less than ), GT (for is greater than ), EQ (for is equal to ), or MI (for more information required ) in the blanks provided. 1. The solubility of CH 3 OH in water is (1) the solubility of CH 3 Cl in water at 25 C. 2. The osmotic pressure of a solution at 75 C is (2) the osmotic pressure of the same solution at 25 C. 3. The freezing point depression of a1maqueous solution of C 2 H 5 OH is (3) the freezing point depression of a1maqueous solution of NaCl. 4. The boiling point of a 1 m aqueous solution of NH 4 NO 3 is (4) that of a 1 m aqueous solution of KBr. 5. The boiling point of a1maqueous solution of glucose (C 6 H 12 O 6 ) is (5) the boiling point of a1msolution of glucose (C 6 H 12 O 6 ) in ethanol. B. (12 points) The questions in this part are not related to each other. 1. What is the normal boiling point of a m aqueous solution of aluminum chloride? 2. A solution is made up of 60.0 g of urea (CO(NH 2 ) 2 ); M = 60.0 g/mol) and g of water. What is the vapor pressure of the solution at 25 C? The vapor pressure of pure water at 25 C is 23.8 mm Hg. 3. What is the molarity of a 10.0% (by mass) solution of NaCl (d = 1.08 g/ml)? (Your answer should have 3 sig figs.) 1

2 C. (10 points) Consider Figure 1 in the sheet with figures. Figure 1 is a concentration vs time plot of 2 reactions A and B. Write your answers on the blanks provided 1. Which reaction is slower? 2. Are both reactions zero-order reactions? 3. Which reaction has the larger k? 4. If reactions A and B are the same reaction, which one is done at a lower temperature? 5. If reactions A and B are the same reaction done at the same temperature but where one has a catalyst and the other does not, which reaction has the catalyst? D. (6 points) Consider the following reaction: A+2B C+D The following facts are known about the reaction: (1) The rate of the reaction increases 4-fold (quadrupled) when [A] is doubled while [B] is kept constant. (2) The rate of the reaction does not change when [B] is doubled while [A] is kept constant. 1. Write the rate expression for the reaction. 2. What is k for the reaction when [A] = M, [B] = M and the rate is M/hr? E. (8 points) Consider the following zero-order decomposition, where k = M/s: 2A products 1. What is the half-life of the decomposition when [A] = M? 2. How long will it take to decompose a 4.00 M solution of A to 2.75 M? 2

3 F. (14 points) Consider the following system at 25 C. 2 A (g) B (g) + 3 C (g) In an experiment at 25 C, the initial partial pressures are: P A = 3.00 atm P B = 0.00 atm P C = 4.00 atm. 1. In which direction will the system (as written) move to reach equilibrium? (Draw a circle around your answer) forward reverse 2. At equilibrium, the partial pressure of A is determined to be 2.50 atm. a. What are the equilibrium partial pressures for B and C? P B = P C = b. What is K for the reaction at 25 C? 3. At another temperature, K for the reaction is 1.6 x Initially P A = 1.00 atm. What are the partial pressures for B, and C at equilibrium? (You may assume that the change in pressure is negligible when compared to the original pressure.) P B = P C = G. (8 points) Consider Figure 2 in the sheet with figures. Figure 2 is a concentration vs time plot of a reaction involving species X, Y and Z. Write your answers on the blanks provided 1. Is compound X a product? 2. How many hours does it take to establish equilibrium? 3. Would the presence of a catalyst increase the equilibrium concentrationn of X? 4. Do the reactants combine in a 1:1 ratio? 3

4 II. (70 points) This part of the final corresponds to Exam II. It covers the material in Chapters 13, 14, and 15. A. (5 points) What is the ph of a solution prepared by adding g of Ba(OH) 2 (M = g/mol) to enough water to make ml of solution? B. (20 points) Consider acetic acid (HC 2 H 3 O 2 ) whose K a is 1.8 x What is [H + ] for a 0.10 M solution of acetic acid? 2. What is the percent dissociation of a 0.10 m solution of acetic acid? 3. What is [H + ] for a solution made up of 0.10 M acetic acid and 1.0 M sodium acetate? 4. Write a balanced net ionic equation for the reaction between acetic acid and sodium hydroxide. 5. What is the ph of a solution prepared by adding 25.0 ml of 0.10 M NaOH and 30.0 ml of 0.20 M acetic acid? 4

5 C. (10 points) Rank the folowing 0.10 M solutions in terms of increasing ph. NaCl NH 4 Cl Ba(OH) 2 HNO 3 NaF < < < < D. (20 points) Consider the titration of 50.0 ml of M HCl with M NaOH. 1. What is the ph before any NaOH is added? 2. How many ml of NaOH are required to reach the equivalence point? 3. What is the ph at half-neutralization? 4. What is the ph at the equivalence point? 5. If 5.0 ml of 0.10 M NaOH were added past the equivalence point, what is the ph of the resulting solution? 5

6 E. (15 points) Consider Fe(CN) 3 6. Its K f value if 4 x The metal ion is known to have one unpaired electron. 1. What is the abbreviated electron configuration for the metal ion? 2. Write the electron distribution for both the high spin and low spin models for the octahedral complex. high spin model 3. Does the CN ligand produce a weak field ligand? low spin model 4. Can we expect Fe(CN) 3 6 to be colorless? 5. What is [CN ] when [Fe(CN) 3 6 ]=[Fe 3+ ]? 6

7 III. (70 points) This part of the final corresponds to Exam III. It covers the material in Chapters 16, 17, and 18. A. (10 points) The questions on this part are not related to each other. 1. Write the Ksp expression for calcium phosphate. 2. Zn(OH) 2 hasak sp of4x What is the ph of Zn(OH) 2 in pure water? B. (11 points) Silver bromide has Ksp =5x Ag(NH 3 ) + 2 has K f =1.7x What is the solubility (in mol/l) of AgBr in 2.0 M sodium bromide? 2. What is the equilibrium constant for dissolving silver bromide in ammonia? 3. What is the solubility (in mol/l) of silver bromide in 2 M ammonia? 7

8 C. (9 points) Consider the following reactions: (a) NH 3 NO 3 (s) N 2 O (g) + 2 H 2 O (g) (b) CO 2 (g) CO 2 (s) (c) 2 H 2 (g) + O 2 (g) 2H 2 O(l) (d) 2 H 2 (g)+o 2 (g) 2H 2 O (g) 1. Which reaction has the highest S? 2. Which reaction has the lowest S? 3. Is S for reaction (c) greater than or less than S for reaction (d)? circle one: greater than less than D. (8 points) Consider the following reaction carried out at 25 C. Al 2 O 3 (s) + 2 Fe(s) 2Al(s)+Fe 2 O 3 (s) where H = kj and S = kj/k 1. Calculate the G at 25 C. 2. Calculate the temperature at which this reaction reaches equilibrium. 8

9 E. (6 points) For the graph below: 1. Draw a line on the graph for an exothermic reaction that is spontaneous at all temperatures. 2. Using the relationship between G, H, and S, identify the slope and the y-intercept of your line. G (kj) Temperature (K) F. (10 points) Use the following data to answer the next 5 questions: Al 3+ (aq) + 3 e Al (s) E red = 1.68 V Fe 2+ (aq) + 2 e Fe (s) E red = V Tl + (aq) + e Tl (s) E red = V Cu 2+ (aq) + 2 e Cu (s) E red = V Au 3+ (aq) + 3e Au (s) E red = V Fill in the blanks with the best word or phrase from the following list (you may use a word more than once): greater than less than equal to more information is needed true false electrolytic voltaic oxidation reduction Nernst Faraday. 1. Cu Cu 2+ Tl + Tl is a(n) (1) cell. 2. The E ox for gold metal is (2) 1.00 V. 3. E red for Al3+ is (3) E ox for Cu. 4. It is (4) that iron metal can reduce gold(iii) ion. 5. The ability of Al to reduce Cu 2+ is (5) the ability of Fe to reduce Cu 2+. 9

10 G. (10 points) Consider the following reaction at 25 C where E = V 2 MnO 4 (aq) + 5 Cu (s) + 16 H+ (aq) 2Mn 2+ (aq) + 5 Cu 2+ (aq) + 8 H 2 O 1. What is n? 2. What is Q for the following conditions: [MnO 4 ] = 2.34 M, ph = 4.0, Cu = 2.0 g, [Mn2+ ] = M, [Cu 2+ ] = M 3. What is E under these conditions at 25 C? H. (6 points) An electrolytic cell is made up of a g strip of copper in M Cu(NO 3 ) 2 and a g strip of Cr in M Cr(NO 3 ) 3. The overall reaction is: 3 Cu (s) + 2 Cr 3+ (aq) 3Cu 2+ (aq) + 2 Cr (s) E o = V An external battery provides 3 amperes for 70 minutes and 20 seconds with 100% efficiency. 1. How many coulombs are supplied by the battery? 2. What is the mass of the copper strip after the battery is disconnected? 10

11 IV. (40 points) This part of the final corresponds to Chapter 19. A. (14 points) Fill in the blanks with the best symbol or word from the following list (you may use a word more than once): 0 1 e 1 1 H 2 2 H 0 1 e fusion 1 0 e 4 2 He 1 0 n 0 1 n fission 1. A beta particle 2. an alpha particle 3. a positron 4. a neutron 5. a proton 6. A reaction between 2 small nuclei to produce a larger one, releasing energy in the process. 7. The splitting of a heavy nucleus by a neutron into 2 lighter nuclei accompanied by a release in energy. (6 points) Write your answer to the question in the blank provided. 1. The result of an alpha emission by U. 2. The result of a beta emission by Pa. 3. The result of the bombardment of Al with an alpha particle where only one nucleus is produced. B. (10 points) Given the following nuclear masses: 10 neutron = g proton = g 5 B = g 1. Calculate the mass defect of 10 5 B. 2. Calculate its binding energy. 11

12 C. (5 points) Y has a rate constant of 2.6 x 10 6 s 1. What is the activity of a 1.00 mg sample in Ci? (Assume a mass of 87 g for the nucleus.) D. (5 points) A wooden artifact gives 9.0 disintegrations/min/g. The C 14 activity from a freshly cut down tree is observed to be 15 disintegrations/min/g. The half-life of C-14 is 5730 years. How old is the artifact? 12

13 BONUS (15 points) All or nothing. The bonus should be done only after you have completed the main part of this exam and checked your work for errors. The time allotted for this exam does not include time for the bonus. For the indicator phenolphthalein (In ), HIn is colorless and In is red. The dissociation constant for HIn (K a ) is 1.0 x A colored endpoint for the indicator occurs when the concentration of the colored species is 10 times that of the colorless one. If one has a colorless solution of 50 ml phenolphthalein, how much 0.10 M NaOH solution would one have to add to make it red? (Assume that the OH consumption of the phenolphthalein is negligible.) 13

Chem 128, Exam III April 25, 2002

Chem 128, Exam III April 25, 2002 I. (41 points) A. (4 points) Write the equilibrium equation and K sp expression for Co 2 S 3. Pay attention to chemical state designations, charges and stoichiometric coefficients! equilibrium equation: