Name: page 1/10 Chemistry 125/126, Exam 1 Tuesday, March 16, 2010 Welcome to the first hourly exam for Chemistry 125/126. This exam consists of 6 questions worth a total of 75 points plus a bonus question worth for a possible total of 78 points. It is 10 pages long; 8 pages of questions including a bonus question and periodic tables (page 10). 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. Save any questions you have during the exam for Prof. Kerner. The GSIs have been told not to answer questions during the exam. Graded exams will be available starting tomorrow. If, after checking the exam key, you feel there was a grading mistake, turn your exam in with a statement describing the mistake to Prof. Kerner s mailbox in 1500o chem. All re-grades must be turned in by noon, Monday, March 29, 2010. Course Information Section GSI Section GSI Section GSI 127 T AM Kevin Hartman 191 T PM2 Zhenxin Lin 237 Th PM2 Blake Erickson 129 T AM Rahul Rattan 195 T PM2 Shuwen Sun 239 Th PM2 Russell Bornschein 131 T AM Lauren Soblosky 199 T PM2 Heidi Pedini 241 Th PM2 Joshua Skodack 135 T PM1 Zhenxin Lin 233 ThPM1 Lauren Soblosky 251 F PM1 Akiko Kochi 137 T PM1 Rahul Rattan 225 Th PM1 Shuwen Sun 253 F PM1 Kevin Hartman 139 T PM1 Rachel Barnard 227 Th PM1 Blake Erickson 119 F PM2 Akiko Kochi 141 143 T PM1 T PM2 Russell Bornschein Joshua Skodack 231 235 Th PM1 Th PM2 Rachel Barnard Heidi Pedini 123 F PM2 Alaina DeToma For use by grader Page Points Score Name: 2 10 3 12 GSI: 4 13 5 10 Section: 6 09 7 13 e-mail: 8 08 9 (+03) 10 75 (+03)
Name: page 2/10 Question 1 (16 points) deals with your team s study of the reaction that occurs when you mix 10 ml of 0.10 M solutions of silver chlorate and cobalt sulfate: AgClO 3 (aq) + CoSO 4 (aq) precipitate (white) A. Table 1 below shows the electron configuration of Ag. Fill in Table 2 with the electron configuration of the metal ion Ag + and record if the silver chlorate solution is colored or colorless based solely on the cation's electron configuration. Table 1. Configuration Table 2. Configuration Colored or colorless? Ag [Kr] 4d 10 5s 1 Ag + 2 points B. You hypothesize that Ag+ is critical to the formation of the precipitate. You design a reference blank test to directly test your hypothesis. Indicate the specific reagents you mix for your reference blank test: +? Indicate (circle) if a reaction or no reaction will occur if your hypothesis is confirmed: no reaction reaction 2 points C. Your team performs further tests indicated below in an attempt to identify the precipitate formed in the reaction of AgClO 3 (aq) + CoSO 4 (aq). Tests: (Note: all tests use 10 ml of 0.10 M solutions) : I. AgClO 3 (aq) + CoI 2 (aq) yellow precipitate. 2. AgClO 3 (aq) + Co(NO 3 ) 2 (aq) no reaction. 3. AgClO 3 (aq) + K 2 SO 4 (aq) white precipitate Circle any test that is appropriate and, by itself, confirms the identity of a species that is CRITICAL to the formation of the precipitate in the AgClO 3 (aq) + CoSO 4 (aq)reaction. 1 2 3
Name: page 3/10 D. Another team (Z) performed the reaction under investigation and observed no precipitate formation. Which one or more of the statements below is a possible explanation for the difference in results? Check ( ) either the Possible or Not Possible column. Statement Possible Not Possible 1. Team Z mixed 0.004 M (rather than 0.10 M) solutions of AgClO 3 and CoSO 4. 2. Team Z mixed 1 ml (rather than 10 ml) of 0.10 M solutions of AgClO 3 and CoSO 4. 3. There was a small amount of hexane present in the reaction flask used by Team Z. 6 points Question 2 (12 points) asks you to make predictions based on your knowledge of the relationship of structure and properties and reactivity. Periodic tables are on page 10 for your reference. A. Predict the relative oxidizing agent strength of K +, Ca 2+, and Br 2. Strongest weakest oxidizing agent > > B. Predict the relative solubility of BaCl 2, SrCl 2, and CaCl 2 in water. Most soluble least soluble > >
Name: page 4/10 C. (Circle your choice) that will correctly complete each statement below (2 points each). 1. An aqueous solution of ( Cu(NO 3 ) 2 CuNO 3 Ni(NO 3 ) 2 ) is colorless. 2. A spontaneous redox reaction will occur between Br - and ( Na F 2 At 2 ). 3. The ionic radii of Mg 2+ is greater than that of ( Al 3+ Na + Ba 2+ ). Question 3 (7 points) deals with your preparation of solutions of Rb 2 S. A. A team is to prepare 0.10 M rubidium sulfide, Rb 2 S. A team member transfers 2.03 g Rb 2 S to a beaker. She then uses a buret to deliver 100 ml of water into the beaker containing the rubidium sulfide and stirs the contents. Circle the concentration of the resulting rubidium sulfide solution: Less than 0.10 M equal to 0.10 M greater than 0.10 M B. You prepare 0.10 M Rb 2 S from 50 ml of 0.50 M Rb 2 S. What volume (ml) of water did you add to 50 ml of 0.50 M Rb 2 S? For maximum credit, you must record all steps in your calculations. Numerical answers alone will not receive credit. ml of water = 4 points ml
Name: page 5/10 Question 4 (17 points) deals with the absorption spectra below of an indicator in acidic and basic solution. The path length = 1cm and the concentration of the indicator solution = 2.0 x 10-2 M. Absorbance 0.7 0.6 0.5 Purple Blue Green Yellow Orange Red acidic basic 0.4 0.3 0.2 0.1 0 400 450 500 550 600 650 700 wavelength (!) in nm A. What is the observable color of the indicator in basic solution? Circle the best choice for the color of the indicator in basic solution: orange blue green purple B. Based on the above spectra, indicate whether each of the following statements are correct or incorrect (place an "X" under correct or incorrect). Statement Correct Incorrect 1. Transmission of visible light by the acidic solution is greater at λ 600nm than at λ 450 nm. 2. Calibration graphs are produced at λ = 450. The slope of the calibration line for basic solutions is greater than the slope for acidic solutions. 2 points each C. Use your knowledge of the Beer-Lambert law and the above absorption spectra to determine the concentration of an acidic solution of the indicator of unknown concentration. 1. A diluted acidic indicator sample of unknown concentration has an absorbance of 0.25 at λ 575. What is the concentration (M) of the diluted sample? For maximum credit record all calculations. Numerical answers alone will not receive credit. M of diluted sample =
Name: page 6/10 2. The diluted sample (C1 above) was prepared by mixing 6.00 ml of water and 2.00 ml of the acidic indicator of unknown concentration. What is the concentration (M) of the undiluted acidic indicator sample of unknown concentration? M of undiluted sample = D. The concentration (M) that you determined was subsequently found to be too high (that is, the true concentration was lower than indicated by the numbers in question C2). Specify if each statement below could be a source of error that could cause the concentration to be too high. Put a in either the Correct or Incorrect column. Statement Correct Incorrect 1. The volume of water added to dilute the sample was actually 4.0 ml, not the recorded 6.0 ml. 2. The water used to dilute the sample contained an impurity that absorbs at λ=575 nm. 3. The sample holder had a shorter path length than the sample holder used to produce the absorption spectrum. (6 points)
Name: page 7/10 Question 5 (13 pts) deals with your redox study of the metal ions Pt 2+, Ag +, Cu 2+, and Co 2+ and their metals. The table below shows your observations except for Pt (Platinum): Pt Cu Co Ag Pt 2+ reaction Reaction reaction Cu 2+ Reaction no reaction Co 2+ no reaction no reaction Ag + reaction reaction A. Based on the observations rank the oxidizing agent strength of Ag +, Cu 2+, and Co 2+. strongest oxidizing agent weakest oxidizing agent > > B. Indicate (check, ) if each statement below is correct or not correct. Statement Correct Not Correct 1. If Pt 2+ is a good oxidizing agent then most likely Pt is a poor reducing agent. 2. Based on the observations, Ag is a better reducing agent than Co. 3. When Co is added to a solution of Pt(Cl) 2, Pt (s) forms. For use by grader 0-6 points C. You add a strip of Pt to a solution of AgNO 3. Does a redox reaction occur? Why or why not? Circle "yes" or "no" to indicate if Pt will react with 0.10M AgNO 3 : Yes. a reaction will occur No reaction will occur 2 point Explain the basis of your decision for "reaction" or "no reaction". Use the term oxidizing agent or reducing agent in your explanation. 2 points
Name: page 8/10 Question 6 (8 points) asks you to predict redox reaction products from the following information: Best reducing agent K > Zn > Sn 2+ > I - > Br - > Cl - Worst reducing agent A. You add a few crystals of SnCl 2 to a test tube (I) with water. The crystals dissolve. Complete a balanced equation for the dissociation of SnCl 2(s) into ions in water: SnCl 2 (s) + water B. You add a few crystals of KI to a different test tube (II) with water. The crystals dissolve. You mix the contents of test tubes II and I and add hexane and shake the mixture. Based on the reducing agent strengths indicated above, determine if a redox reaction occurs. Identify (circle) any species present in the hexane phase or indicate (circle) NONE. NONE Cl 2 Cl - I 2 I - Sn 2+ K + C. If SnCl 2(aq) is added to AgNO 3(aq) a reaction occurs where Ag (s) is formed. Rank Sn 2+ and Ag as reducing agents: Best reducing agent > Worst reducing agent 2 points
Name: page 9/10 BONUS QUESTION ( or zero) The dark color of an iodine stain can be removed by rubbing a dampened vitamin C tablet over the surface (such as fabric) of the stain. The color of the iodine vanishes. This question refers to the additional observations below: Some additional observations: If you rinse out the vitamin C treated fabric, the iodine stain will not reappear. If you do not rinse out the fabric the color of the iodine stain can reappear even before the fabric is washed. If you do not rinse out the fabric, and add bleach, the stain will reappear. Provide an answer to each of the three questions below using a maximum of 10 words. 1) Rinsing is critical. What does rinsing remove? 2) Would it be preferable to rinse the vitamin treated stain area with water or hexane. Why? 3) Is bleach an oxidizing or reducing agent?