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____________________________________page Name: 1/9 Chemistry 125, Exam 2 Tuesday, December 13, 2005 Welcome to the second hourly exam for Chemistry 125/126. This exam consists of 9 questions worth a total of 75 points plus a bonus question worth 3 points for a possible total of 78 points. It is 9 pages long; 6 pages of questions (pages 2-7), bonus question (page 8) and periodic tables (page 9). A few notes about the exam. To receive credit, your answers must be placed in the indicated spaces or boxes. If calculations are required to obtain an answer, show your calculations; you will not receive credit for numerical answers alone. Do not write in any box marked "For use by grader". The exam should take about l hour, however, you may use up to 1.5 hours to complete the exam. All exams must be turned in at 7:45 pm. If you have any questions during the exam, save them for Prof. Kerner. The GSIs have been told not to answer questions during the exam. Your graded exams will be available in the Science Learning Center tomorrow between 10am and 3pm. If, after consulting the exam key, you feel there was a mistake in grading, please turn your exam in to Prof. Kerner (mailbox, 1527 chem) with a statement describing the mistake. All regrades must be turned in by noon, Monday, December 19, 2005. Course Information Section GSI Section GSI Section GSI 111 113 115 117 121 123 125 127 129 131 133 135 137 139 141 143 M AM M AM M AM M AM T AM T AM T AM T AM T AM T PM T PM T PM T PM T PM T PM F AM Yi Zhu Akash Bhattacharya Ron Smith Jingjing Li Woj Zalas Chen Li Andrew Higgs Matthew Remy Christopher Avery BoWang Laura Zimmerman Andrew Higgs Qinyi Yan Marie van Staveren Christopher Avery Akash Bhattacharya 145 147 149 239 241 243 245 247 249 251 253 255 257 259 261 263 F AM F AM M AM W PM Th PM W PM W AM W PM W PM W PM W PM W AM W AM W PM Th AM Th AM Rodney Agayan Matthew Remy Shujiang Cheng Meghan Wagner Katherine Hersberger Shujiang Cheng Jingjing Li Maura Perry Kathryn Dooley Brantley Carlson Cornelius Kristalyn Rodney Agayan Mark Roll Laura Zimmerman Amy Payeur Tom Horvath 265 267 271 273 275 277 279 281 283 285 287 289 341 343 351 353 Th AM Th AM Th PM Th PM Th PM Th PM Th PM F AM F AM F AM F AM F AM F AM F AM F AM F AM Katherine Hersberger Woj Zalas Meghan Wagner Maura Perry Kathryn Dooley Bo Wang Qinyi Yan Mark Roll Tom Horvath Cornelius Kristalyn Neil Kamdar Chen Li Brantley Carlson Neil Kamdar Ron Smith Yi Zhu Name: GSI: Section: For use by grader Page Points Score 15 2 12 3 10 4 14 5 12 6 12 7 8 Total (+03) 75 (+03) e-mail: Page 1/9 Name: ____________________________________page 2/9 Question 1 (9 points) asks you to make pH predictions based on your knowledge of properties and structure. Periodic tables are provided at the back of this exam for your reference. A. Predict the comparative pH of 0.10M solutions of Mg(OH)2, Al(OH)3, and KOH. Lowest pH _________________ 3 points highest pH < __________________ < ________________ B. Predict the comparative pH of 0.10M solutions of Pb(NO3)2, Mg(NO3)2, and Ba(NO3)2. Lowest pH _________________ 3 points highest pH < ___________________ < ________________ C. Predict the comparative pH of 0.10M solutions of Na3PO4, Na2HPO4, and NaH2PO4. Lowest pH _________________ 3 points highest pH < ___________________ < ________________ Question 2 (6 points) asks you to experimentally distinguish between pairs of solid white salt compounds based on their Lewis Acid-Base behavior. The tests that you can perform are: A. Determine the pH values of aqueous solutions of the compounds. B. Determine if the compounds dissolve in 5M NH3. C. Determine if the compounds dissolve in 5M NaOH. Select the one letter (A, B, or C) that corresponds to the one best test for distinguishing between the two compounds given below. Test Comp ounds (A, B, or C) CuCO3 vs BaCO3 KNO3 PbC2O4 vs vs Pb(NO3)2 CaC2O4 For use by grader only 6 points Page 2/9 Name: ____________________________________page 3/9 Question 3 (12 points) refers to the pH values obtained for the following samples: 0.10 M Sample hydrofluoric acid, HF phosphoric acid, H3PO4 sulfurous acid, H2SO3 pH 2.2 2.5 3.3 0.1M Sample hydrocyanic acid, HCN Na3PO4 0.10M NH3 pH 5.0 10.9 11.0 A. (6 points) Use the above pH data to conduct calculations and answer questions (1. 2.): 1. What is the [H+ ] in the solution of 0.10M HF? Show your calculations for complete credit. (3 points) 2. Is HF a strong acid? (Circle one) Yes No (1 point) Why or why not? (2 points) B. (6 points) Use the above pH data and your knowledge of acids and bases to make pH predictions for untested samples. Circle your choice (of word) to correctly complete each statement. 1. The pH of 0.10M H2CO3 is 2. The pH of 0.10M NaCN is ( ( higher higher higher lower lower lower ) than the pH of 0.10M H2SO3. ) than the pH of 0.10M NaF. ) than the pH of 0.10M Na3PO4. For use by grader only 6 points 3. The pH 0.10M (NH4)3PO4 is ( Page 3/9 Name: ____________________________________page 4/9 Question 4 (10 points) deals with your team's titration of an unknown carboxylic acid that is among the compounds listed below: 1. 2. 3. 4. Compound Propenoic acid Butanoic acid Oxalic acid Adipic acid Formula HOOC-CH=CH2 CH3CH2CH2COOH HOOC-COOH HOOCCH2CH2CH2COOH MW 72 88 90 146 A. You titrate 0.180 g of the unknown carboxylic acid using 0.10 M NaOH and the best choice from four available indicators listed below: Indicator pH end point range Congo red Bromothymol blue 3.0 5.0 6.0 7.6 Indicator Thymol blue 1, 3, 5- Trinitrobenzene pH end point range 8.0 9.6 12.0 14.0 1. Indicate (circle) the primary factor that will govern your choice of indicator. Reaction stoichiometry Equivalence point pH Base (NaOH) concentration points 3 2. Indicate (circle) the best choice of indicator . Congo Red Bromothymol Blue Thymol Blue 1,3,5-Trinitrobenzene 3 points B. The unknown is titrated in order to determine its equivalent weight. 0.180 g of the carboxylic acid requires 24.79 mL of 0.10 M NaOH to reach the equivalence point. Based solely on this data, and assuming an uncertainty of 5% in equivalent weight results, circle any compound that may be the unknown. Circle any compound that may be the unknown based solely on the equivalent weight results. Propenoic acid 4 points Butanoic acid Oxalic acid Adipic acid Page 4/9 Name: ____________________________________page 5/9 Question 5 (8 points) deals with aluminum species found in aqueous solutions at varying pH: A. [Al(H2O)6]3 + B. [Al(H2O)5(OH)]2 + C. [Al(OH)6] 3 - D. [Al(H2O)3(OH)3] You perform the procedures below. Fill in the blank spaces to identify the predominant aluminum species formed (A, B, C, or D) and/or indicate your predicted observations. Procedure Observations from start to finish of procedure 1. A white precipitate (ppt) forms. 2. 3. . Predominant Al Species (A, B, C, or D) upon completing procedure 1. 1. To 2 mL 0.1 M Al(NO3)3 , you add 1 mL of 1 M NaOH. 2. You add 1 mL of 1 M NaOH to the resulting mixture (step 1). 3. You add 2 mL of 1M HNO3 to the resulting products of step 2. 2. 3. C. [Al(OH)6] 3- Question 6 (6 points) deals with your investigation of the reaction: 5 mL 0.10 M CaI2 (aq) + 5 mL 0.10 M PdCl2 (aq) Formation of a solid + brown solution Clear and colorless Clear and red You hypothesize that "this is a precipitation reaction where the solid product is PdI2". Which, if any, of the test observations below supports the hypothesis? Put an X in the "YES" column if the observation supports the hypothesis. If the observation does not support the hypothesis put an X in the "NO" column. Test 1. Add 5 mL of 0.10 M Ca(NO3)2 to 5 mL of 0.10 M PdCl2. 2. Add 5 mL of hexane to the product mixture (from the original CaI2 + PdCl2 reaction). 3. Add 5 mL of 5 M NH3 to the product mixture (from the CaI2 + PdCl2 reaction). Observation 1. No reaction. 2. Two phases form. The upper hexane phase is pink in color. 3. No reaction. YES No Page 5/9 Name: ____________________________________page 6/9 Question 7 (6 points) deals with the reactions of 0.1 M CuCl2 and Na2CO3 and 1.0 M NH3. Reactions: 1. CuCl2(aq) + Na2CO3(aq) blue ppt. forms Clear blue colorless 2. Mixture from step 1 + NH3(aq) colorless blue ppt dissolves; solution turns violet-blue. Information: Reactions 1 and 2 are Lewis acid-base (complexation) reactions involving [Cu(H2O)4]2 +. A. What will you observe when you conduct the following test: Reaction 2 product mixture (violet-blue solution) Visual observations of the test results: + 1 M HNO3 Observations? 2 points B. If you reverse the order of addition of reagents to the copper (II) chloride, what will you observe?. Reactions: B1. CuCl2(aq) Clear blue + NH3(aq) colorless Visual observations? ________________________________ ________________________________ 4 points B2. Step B1 mixture + Na2CO3(aq) colorless Question 8 (6 points) deals with your team's "analysis of a reaction" where you add the metal Cd to a yellow-green solution of 0.10 M FeBr2(aq) and observe no apparent reaction. Circle every observation (1and/or 2 and/or 3) that can help confirm that NO reaction took place. Observation 1. 2. 3. Add hexane to the reaction mixture and the hexane remains clear and colorless. Add Cd to 0.10 M FeBr3 and observe a reaction. Add Fe to 0.10 M CdBr2(aq) and observe a reaction. Page 6/9 Name: ____________________________________page 7/9 Question 9 (12 points) deals with your analysis of the reaction: FeCl3 (aq) A. + CrBr2 (aq) Change of color + violet precipitate You hypothesize that "the products of reaction are Fe2 +, Cl- , Cr2 +, and Br2 ". Assume your hypothesis is correct. If your hypothesis is correct, are the statements below correct or incorrect? Check ( ) the "correct" or the "not correct" column. Statement Correct Not Correct 1. The spectator species in the reaction are Cl and Cr2+. 2. Br2 is a stronger oxidizing agent than Fe3+. 4 points B. You conduct tests (1-2 below) to identify spectator and reactant species in the FeCl3 + CrBr2 reaction. A goal is to identify species causing the color change and the violet precipitate. (For complete credit, indicate the correct symbol and charge on any species you refer to.)___ 1. FeCl3 (aq) + Cr(NO3)2 (aq) Change of color + violet precipitate (same properties as the reaction under analysis) Change of color + no precipitate (same color change as the reaction under analysis) 2. Fe(NO3)3(aq) + CrBr2 (aq) Test 1 results indicate that: 3 points Test 2 results indicate that: 3 points C. You carry out additional tests (3 - 4) in an attempt to understand the FeCl3 + CrBr2 reaction: 3. NaCl (aq) 4. FeCl3 (aq) + CrBr2 (aq) + NaBr (aq) No reaction. No reaction. Based on tests 1-4, is your original hypothesis correct? Indicate (circle) your answer. Yes No 2 points Page 7/9 Name: ____________________________________page 8/9 BONUS QUESTION (3 points or zero) Baking powder contains sodium bicarbonate, NaHCO3, combined with the double sulfate salt, NaAl(SO4)2. When added to moist dough, small bubbles of CO2 give the finished baked product a light and porous texture: ____?____ + H3O+ CO2(g) + 2 H2O 1. Fill in the missing species in the equation for the reaction of baking powder: _______________ + H3O+ CO2(g) + 2 H2O 2. Identify the species in the baking powder that is critical to the production of the hydronium ions, when the baking powder is added to moist dough: ________________ 3 points or zero Page 8/9

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