Chem 128, Final Exam May 5, 2004

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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. For parts A, C, D, E show all your work no matter how trivial. A. (20 points) Consider chloroform, CHCl 3. The information given below may help you solve problems 1 5. molar mass = g/mol boiling point = 61.7 C density = 1.49 g/ml vapor pressure at 45 C = 431 mm Hg 1. A solution is prepared by adding 14.6 g of a non-electrolyte (MM = g/mol) to 132 ml of CHCl 3. What is the molality of the resulting solution? 2. The solution in #1 boils at 64.4 C. What is the boiling point constant for CHCl 3? 3. Another solution is prepared by dissolving an electrolyte in CHCl 3 to make a m solution. The solution boils at 66.7 C. What is i for the electrolyte? 4. Naphthalene is added to CHCl 3 at 45 C. The vapor pressure of the resulting solution is 403 mm Hg. What is the mol fraction of naphthalene in the mixture? 5. Another solution is prepared where naphthalene (MM = g/mol) is added to 0.50 mol of CHCl 3 at 45 C. How many grams of naphthalene are required so that the mole fraction of naphthalene in the solution is 0.20? 1

2 B. (6 points) Which would be more soluble in water? (Write your answers on the blanks provided.) 1. CH 3 OH or C 10 H 21 OH 2. CBr 4 or CHBr 3 3. NaI or C 2 H 4 I 2 C. (12 points) The peroxysulfate ion reacts with iodide ion in aqueous solution according to the following equation: S 2 O 2 8 (aq) + 3 I (aq) 2SO 2 4 (aq) + I 3 (aq) The following data are obtained at a certain temperature: Expt. [S 2 O 2 8 ] [I ] rate (M/min) x x x x What is the order of the reaction with respect to the reactants. [S 2 O 2 8 ] [I ] 2. Write the rate expression for the reaction 3. What is the rate constant for the reaction? Do not forget the units 2

3 D. (15 points) Consider the zero-order decompositon of HI on a gold surface. HI (g) 1 2 H 2 (g) I 2 (g) 1. Write the rate expression for the reaction. 2. At a certain temperature, it takes 16 s for the pressure of HI to go from 1.00 atm to atm. What is the rate constant for the reaction at that temperature? 3. At another temperature, the rate constant is determined to be atm/s. How long will it take (in seconds) to decompose 28.8% of HIif one starts with 1.00 atm of HI? 4. At the temperature of the experiment in #3, what is the half-life of HI at a pressure of atm? 5. At the same temperature as #3, what is the rate of decomposition of HI when HI is atm? 3

4 E. (17 points) Consider the equilibrium N 2 +O 2 (g) 2NO(g) 1. Show the effect on the partial pressure of N 2 by using the words increases, decreases, orremains the same. 1. adding NO. 2. adding argon gas. 3. adding O 2 2. Calculate Q for the reaction if atm of N 2, atm of O 2, and atm of NO are mixed in a 10.0 L flask. 3. If K for the reaction is , in which direction will the reaction proceed using the partial pressures in #2? Circle the right answer: to the left to the right 4. At the same temperature (K = ), and using the intial pressures in #2, calculate the partial pressures for N 2 and NO at equilibrium. P N2 = P NO = 4

5 II. (70 points) This part of the final corresponds to Exam II. It covers the material in Chapters 13, 14, and 15. A. (10 points) Consider 6 beakers described below. They all have aqueous solutions. Beaker A = 0.10 M HNO 3 Beaker B=0.10MNH 4 Cl Beaker C=0.10MCH 3 NH 2 Beaker D = 0.10 M NaOH Beaker E = 0.10 M KBr Beaker F = 0.10 M NaHCO 3 Fill in the blanks with <, >,=,ormi(more information) 1. ph of Beaker A ph of beaker D 2. ph of Beaker B ph of beaker C 3. ph of Beaker D ph of beaker E 4. ph of Beaker E ph of beaker F 5. ph of Beaker D ph of beaker F B. (10 points) Consider the solutions below. Each solution has a volume of 1.00 L. Write B if the solution is a buffer and N if the solution is not a buffer mol HNO mol NaNO mol HNO mol KF mol HNO mol NaOH mol NaNO mol NaOH mol HCl mol NaNO mol NaOH mol HCl. C. (10 points) Write your answers to the following questions in the blanks provided. 1. What is the oxidation number of chromium in the complex ion[cr(oh) 2 (C 2 O 4 ) 2 ] 3? 2. Write the abbreviated electron configuration Mn Write the formula for the phosphate salt of [Pt(NH 3 ) 4 ] How many different geometric isomers can one draw for [Co(H 2 O)(NH 3 )(en) 2 ] 2+? 5. What is the geometric shape of cis-[ni(oh) 2 (Cl) 2 ]? 5

6 D. (10 points) Consider the titration of HF (K a =6.9x10 4 ) with KOH where KOH is the titrating agent. Write your answers on the blanks provided. 1. What species are present at the equivalence point? 2. What is the ph at half-neutralization? 3. What would you expect the ph to be at the equivalence point? 1, 4, 8, or 14 (Choose one of these values and write on the blank provided.) E. (14 points) The problems for this part are not related. 1. What is the ph of a 0.45 M solution of HClO 4? 2. What is the ph of a solution obtained by adding 1.71 g of Ba(OH) 2 (MM = 171 g/mol) to 125 ml of 0.20 M NaOH. Assume no volume change. 3. What is K b for F?(K a forhf=6.9x10 4 ) 4. What is the K a for a weak acid (HA) if a M solution has a ph of 1.21? 6

7 F. (8 points) Consider the HF/F buffer. (K a forhf=6.9x10 4 ) 1. What is the ph of the buffer if 21.0 g of NaF (MM = 42.0 g/mol) are added to ml of 1.00 M HF. 2. To the buffer in (1) is added 4.00 g of NaOH (MM = 40.0 g/mol). What is the ph of the resulting solution? G. (8 points) Consider the complex ions [Cr(A) 6 ] 3 and [Cr(B) 6 ] 3 where A and B are ligands. The following reaction occurs. [Cr(A) 6 ] 3 (aq) + 6 B (aq) [Cr(B) 6 ] 3 (aq) + 6 A (aq) Before reaction occurs, the intial concentrations of the reactants are [Cr(A) 6 ] 3 = M; B = 0.10 M. After equilibrium is established, [B ] = 0.04 M. What are the equilibrium concentration for [Cr(A) 6 ] 3, [Cr(B) 6 ] 3, and A? [Cr(A) 6 ] 3 =, [Cr(B) 6 ] 3 =, [A ]= 2. What is K for the reaction? 7

8 III. (70 points) This part of the final corresponds to Exam III. It covers the material in Chapters 16, 17, and 18. A. (14 points) Consider aluminum phosphate (AlPO 4 ) with K sp =1.0x Write a balanced net ionic equation for the equilibrium established between the solid and its corresponding ions in solution. 2. What is its K sp expression? 3. What is the molar solubility (mol/l) of AlPO 4 4. If NaNO 3 is added to a saturated solution of AlPO 4,[Al 3+ ] (circle the right answer) increases decreases remains the same 5. If AlCl 3 is added to a saturated solution of AlPO 4, [PO 3 4 ] (circle the right answer) increases decreases remains the same B. (9 points) Consider CaCO 3 (K sp = 4.9 x 10 9 ). A solution is made up by adding mol of Ca 2+ and mol of CO 2 3 to ml of water. (Assume no volume change.) 1. How many moles of CaCO 3 precipitate are obtained? 2. What is [CO 2 3 ] after equilibrium is established? 3. What is [Ca 2+ ] after equilibrium is established? 8

9 C. (10 points) Solid aluminum hydroxide, Al(OH) 3,(K sp =2x10 31 ) can be dissolved in HCl. 1. What species are present after reaction is complete? (Do not include water.) 2. Calculate K for the reaction. 3. What is the molar solubility of Al(OH) 3 at ph 4? D. ( 6 points) Consider the following reaction: A 2 (g)+b 2 (g) 2AB(s) H = 100kJ One can state with accuracy that (Fill in the blanks with <, >, = or MI): S 0; G 0;K 1 E. (10 points) Ethanol can be made by the fermentation of glucose, C 6 H 12 O 6. C 6 H 12 O 6 (aq) 2C 2 H 5 OH (l) +2CO 2 (g) At 25 C: H = 82.4 kj; G = kj; S o for CO 2 (g) = J/mol-K; S o for C 2 H 5 OH (l) = J/mol-K 1. Calculate S for the reaction at 25 C. 2. What is S o for C 6 H 12 O 6 (aq) 9

10 F. (11 points) Use the table of standard reduction potentials below to answer the following questions. Mg 3+ (aq) + 2 e Mg (s) E red = V Cd 2+ (aq) + 2 e Cd (s) E red = V Ni 2+ (aq) + 2 e Ni (s) E red = V Cu 2+ (aq) + 2 e Cu (s) E red = V Ag + (aq) + e Ag (s) E red = V 1. What is the strongest reducing agent 2. What is E o for the cell: Cu Cu 2+ Ag + Ag 3. Is the cell a voltaic cell? 4. For the Ni 2+ Ni half cell, a current of 3.5 amps flows through the cell for 22 minutes. How many grams of metal are deposited? G. (10 points) A voltaic cell has E o = V and the following reaction 2Ag + (aq) + Cd (s) 2 Ag (s) + Cd 2+ (aq) 1. What is G for the reaction? 2. What is E at 25 C when [Ag + ] = 0.35 M and [Cd 2+ ] = 1.5 M? 10

11 IV. (40 points) This part of the final corresponds to Chapter 19. The following table of nuclear masses (in amu) may be useful: 1 0 n = H = H = He = Rn = Po = A. (14 points) Consider the reaction: Rn He + 84 Po 1. What is the change in mass when 1.00 g of Rn-222 decays? 2. How much energy (in kj) is associated with the decay of 1.00 g of Rn-222? B. (5 points) What is the mass defect of 4 2 He? 11

12 C. (21 points) U-238 has a half life of 4.5 x 10 9 years. 1. If one starts with 1.50 kg of U-238, how many kg are left after 2.00 x 10 9 years? 2. If the end product of the decay is Pb-207, what mass of Pb-207 is produced by this decay? 3. How long will it take for 90.00% of 1.00 kg of U-238 to decay to Pb-207? 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. Uranium in water decays to form Zn 2+ and Sm by fission. Uranium has a half life of7x10 8 years. The zinc ions complex with water and act as a weak acid according to the following equation: Zn(H 2 O) 2+ 4 (aq) H + (aq) + Zn(H 2 O) 3 (OH) + (aq) K a = Assuming that this is the only radioactive decay with any significance and that neither Sm nor U acts as a weak acid, how many years will it take a 1.00 M solution of U to achieve a ph of 6.0? 13

Chem 128, Final Exam May 11, 2003

Chem 128, Final Exam May 11, 2003 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