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209exam1 - CHEM 209 Final Exam NAME(30 POINTS(a A Chemistry...

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Unformatted text preview: CHEM 209 _ Final Exam ' 2/20/06 NAME: . (30 POINTS) (a) A Chemistry major obtains a high-paying summer job in an analytical laboratory and is given the task of determining the ethanol content (int/o] %) of a commercial mouthwash sample. He/she decides to do this by gas chromatography because the experiment was so enjoyable in Chem 209. Accordingly, the student-takes exactly 2.0 mL of the mouthwash sample and dilutes this to 25.0 mL with deionized water. He/she then injects 2.0 uL of the diluted sample into the GC and obtains a chromatogram containing two peaks. The first peak, which corresponds to ethanol, has a retention time of 75 seconds and an area of 216 while the second peak, which corresponds to water, has a retention time of 117 seconds and an area of 1212. The student then makes up several standard ethanol solutions ranging from 0 to 30% . ethanol (vol %), injects 2.0 p]. of each into the GC, and uses the areas of the ethanol peak - to create a calibration curve whose linear regression equation is: ' y= 144.2 x Calculate the ethanol concentration (vol %) of the mouthwash sample. SHOW ALL WORK. . (b) Suppose that chromatogram obtained for an unknown sample contains gig; distinct peaks. What is the minimum number'of different species in the sample? What is the maximum number of different species in the sample? Briefly indicate two reasons why these two numbers might not be the same. 2. (10 POINTS) Write the equilibrium constant expressions for the following chemical reactions: (a) . _ H50; + 13‘ + 31120 '——> HSO4' +3r + 2-H30+ K = .(b) ' P130124, —~——> Pb2+ + 2 cr K ‘= ' 3. (30 POINTS) (a) Write the balanced chemical reaction that occurs during the com metple titration of Na2C03 with HCI. (b) On the graph below, sketch the shape of the titration curve expected for this titration. mL HCl (c) If the Na2C03 weight taken for the titration was 0.4444 gram and the concentration of the HCl solution was 0.12M, calculate the equivalence point volume expected. SHOW ALL WORK CLEARLY. (Formula weights: Na2C03 = 105.99 g/mole; HC] = 36.46 g/mole) 4. (20 POINTS) Draw block diagrams of each of the following analytical instruments, labeling all the important components. (a) a gas chromatograph (GC) (b) a visible spectrophotometer (e. g, the Spectronic 20) 5. (10 POINTS) Calculate the following and SHOW YOUR'WORK: (a) the pH of a 0.0050M NaOH solution. (b) the %T of 'a solution whose absorbance is 0.50. BONUS QUESTION [10 POINTS] Suppose that your teaching assistant, while working on hisfher MS degree In Chemistry discovers a new compound X which might be used to remove mercury Hg“ (which 15 toxic) from contaminated water samples according to the following balanced reaction: The TA tries to determine the numerical value of the equilibrium constant by mixing 50 ml of 0 10M Hg2+ with 50 ml of 0.20M X, allowing2 the reaction to occur and reach equilibrium, and then measuring the resulting Hng2+ concentration as 0. 045M. Help your TA to get his/her degree by completing the calculation needed to evaluate K for the above reaction. SHOW YOUR WORK. ...
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