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1 Chemistry 242 Exam 1 Name February 27, 2013 On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well. Signed If you feel you can t sign this, contact the instructor ( or in person) SHOW YOUR WORK. NO WORK, NO CREDIT (2pts each) Circle the correct answer. 1. Beer's law states that the relationship between the absorbance of a solution and the concentration of the absorbing species in a solution is linear. This relationship is most likely to FAIL when a. the absorbing species is very dilute. b. the absorbing species participates in a concentration-dependent chemical equilibrium. c. a monochromatic laser is used for the light source. d. a mixture of ions are present in the solution evaluated. 2. Two methods of sample preparation that are especially useful for gas chromatography are a. solid-phase microextraction and purge and trap. b. acid digestion, rotoevaporation c. dry ashing and wet ashing. d. microwave assisted extraction and supercritical fluid extraction. 3. When referring to the data obtained from an analysis, the word precision indicates a. how close the mean of the data set is to the true value for the composition of the analyte. b. relative error calculated from the mean of the data set. c. how close the sample mass is to the mass specified in the standard method used. d. how close the individual data points in the data set are to each other. 4. Quantitating an unknown requires comparing to standards. This may be done in one of three ways: use calibration curves; use standard additions; or use internal standards. The method of standard additions would be used when a. a calibration curve is not linear. b. the standard and analyte have similar characteristics that are unaffected by the other material in the sample. c. the quantity of sample analyzed or the instrument response varies from run to run. d. the sample composition is unknown or complex and affects the analytical signal. 5. Which of the following is NOT true regarding a blank solution? a. Blank solutions contain known concentrations of analyte. b. Blank solutions contain all of the reagents and solvents used in the analysis with no deliberately added analyte. c. A blank solution corrects for impurities or interfering species in the analytical samples. d. A blank solution is pure solvent with nothing added. (rest of exam on the back) 1

2 On the rest of the exam: q INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS q USE PAPER PROVIDED -- ONE SIDE OF PAGE q NAME OR INITIALS ON EACH PAGE. q UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS. 6. (10 pts) A sample in a 1.0 mm cell transmits 75.0% of the incident light at 510 nm. If the solution is M, what is its molar absorptivity? 7. (10 pts) An analytical procedure requires 50.0 ml of a standard ppm chromium solution. How many milligrams of primary standard potassium dichromate (K 2 Cr 2 O 7 ) are required to prepare this solution? 8. (10 pts) ml of an iodide solution was treated with ml of M AgNO 3. The solid AgI precipitate was removed by filtration and 1.0 ml of 1% Fe(NO 3 ) 3 was added to the filtrate. This solution required ml of M KSCN to reach a light red endpoint. How many milligrams of iodide were there in the original solution? In this titration, AgSCN precipitates. When all the Ag + has precipitated, the excess SCN -1 reacts with Fe +3 forming the red complex [Fe(SCN)] +2 signaling the endpoint of the titration. When you are finished with this portion of the exam:!sign the honesty pledge or see the instructor!attach this page to your answer sheets & turn them in!pick up the take home exam 2

3 Chemistry 242 Exam 1 Take Home Name (print) February 27, 2013 Due Monday, March 6, 2013 by 5:00 pm On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well. Signed If you feel you can t sign this, contact the instructor ( or in person) SHOW YOUR WORK. NO WORK, NO CREDIT The problems are stated as you might encounter them in the "real world." That means that all the steps needed to arrive at the final answer may not be explicitly stated. Consider all the pertinent aspects of statistical analysis for each problem.! Work on your own, not in groups. Use any books, notes, calculators, and/or computers. You may consult with the instructor for clarification, hints or help in using the computers, but I may not answer all your questions.! Show your work in a logical sequence so I can follow it. Write down enough steps so I can see what you did and locate where you may have made errors.! WORK ON ONE SIDE OF PAGE ONLY! NAME OR INITIALS ON EACH PAGE.! INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS! UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS.! You don t have to use a computer for these problems, but if you do: " Turn in a hard copy of the file including appropriately annotated graph(s) and make sure you show the formulas as you entered them (document your formulas) so I can follow your calculations. " Also the spreadsheet file(s) - include Ex1 and your initials in the spreadsheet file name. 1. Police have a hit-and-run case and need to identify the brand of red auto paint. The percentage of iron(iii) oxide, which gives paint its red color, found in six samples was: 43.15, 43.81, 45.71, 43.23, 42.99, and 43.56%. a. (10 pts) What are the absolute AND relative 95% confidence intervals for this analysis? b. (5 pts) The iron from 0.500±0.002 g of this paint is dissolved into 50 ml of con HNO 3, quantitatively transferred to a Class A 100 ml volumetric flask (±0.08 ml tolerance), and diluted to the mark with deionized water. If the atomic mass of Fe is ± 0.05 g/mole and of O is ± 0.03 g, what is the molarity of Fe +3 in the diluted sample and its absolute standard deviation? Treat all uncertainties as standard deviations. 2. A particular assay method for protein in corn requires a 9.35 g sample to achieve a 1% RSD. a. (5 pts) What sample size is required to achieve a 5% RSD? b. (10 pts) If you run assays using the sample size calculated, how many sample replicates must you run to have 95% confidence that the "true value" is within 2.5% of the average? 3. (10 pts) One candy production line was set to produce a mixture of equal sized yellow and blue jelly beans that was to be 12.5% blue. What sample size (number of beans) must be taken to ensure that the relative standard deviation for the number of blue beans is less than 5%. (rest of exam on back) 1

4 4. The Gibbs Free Energy (ΔG) is a function of the enthalpy (ΔH) and the entropy (ΔS) ΔG = ΔH - TΔS (1) where T is the absolute temperature (Kelvin). Furthermore ΔG for a reaction is related to the equilibrium constant for the reaction by ΔG = -RT ln(k) = RT log(k) (2) where R is the Ideal Gas Constant, J/K mole. Combining equations 1 and 2 we have: RT log(k) = H - T S (3) After rearranging, equation 3 becomes: ΔH 1 ΔS (4) - log(k) = pk = R T R This form of the equation predicts a linear relationship between pk and (1/T) as long as ΔH is independent of temperature (a reasonable assumption for relatively small temperature ranges). Using the data from 0 C to 100 C for the autoionization of water in Table 6-1 (p 129) in your text, Eqn. 4 above, and your regression template: a. (7 pts) Calculate the absolute 95% confidence interval for the ΔH (kj/mole) of the autoionization reaction of water. b. (7 pts) Calculate the relative 95% confidence interval (express as a percent) for the ΔS (J/K mole) of that reaction. c. (6 pts) Appropriately working template with graph. You can do part a and b in the spreadsheet or separately on paper. Clearly show your work. 2

5 Chemistry 242 Exam 2 Name April 8, 2013 On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well. Signed If you feel you can t sign this, contact the instructor ( or in person) (2pts each) Circle the correct answer. 1. Which of the following processes is true regarding the digestion of a solution containing a precipitate? a. Digestion tends to increase particle size of the precipitate. b. Digestion is used to slow down the coagulation of a colloidal precipitate because of the increased energy added to the solution. c. A long digestion is not recommended because the higher temperatures enable impurities to become trapped in the crystal lattice of the precipitate. d. Digestion ensures complete mixing of the reagents. 2. If equal volumes of the following pairs of solutions were mixed, which one would form a buffer? a M HCl and 0.10 M NaCl c M NH 3 and M HCl b M H 2 SO 4 and 0.10 M NaHSO 4 d M HNO 2 and 1.0 x 10-4 M NaNO 2 3. Which indicator would work best for the titration of M sodium nitrite (beaker) with M sulfuric acid (buret)? a. Phenolphthalein b. Bromthymol blue c. Bromcresol green d. Phenol red 4. Which of the following is a reason for endpoint error in a titration? a. Overtitrating the solution b. Titrating the indicator c. Misreading the buret d. All of the above 5. Even in the most precise calculations, activity coefficients are never used in which equation? a. Water K w = [H + ][OH - ] b. Weak acid K a = [H + ][A - ]/[HA] c. Insoluble salt K sp = [M +2 ][A - ] 2 d. Mass balance 0.25 M = [HA] + [A - ] On the rest of the exam: (remainder of the exam on the back) 1

6 SHOW YOUR WORK. NO WORK, NO CREDIT! INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS! USE PAPER PROVIDED -- ONE SIDE OF PAGE! NAME OR INITIALS ON EACH PAGE.! UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS. 6. (20 pts) A sample containing grams of sodium chloride (NaCl) and grams of magnesium chloride (MgCl 2 ). The chloride in the sample was precipitated by the addition of 47.8 ml of a silver nitrate solution. What is the concentration of the silver nitrate solution? 7. (25 pts) A solution containing 30.0 ml of M C 6 H 5 COOH (benzoic acid) is titrated with M NaOH. What is the ph of the solution after the addition of 10.0 ml of NaOH? Explicitly state and check any assumptions you make. 8. (25 pts) What is the ph of a saturated solution of La(OH) 3? Assume the Lewis acid character of the La (III) can be ignored. K sp La(OH) 3 2 x (20 pts) Set up the system of mathematical equations (not just reactions) needed to solve the problem below without making assumptions. DO NOT SOLVE, JUST SET UP THE EQUATIONS. The amino acid glycine (H 2 NCH 2 COOH) has two pk a s, one for the -COOH group (2.35), and one for the -H 3 N + group (9.78). What is the ph of M sodium glycinate? Don t forget the Honesty Pledge 2

7 Chemistry 242 Exam 3 Name print May 8, 2013 On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well. Signed If you feel you can t sign this, contact the instructor ( or in person) Table of Constants J coul R F K mole mole e Nernst Factor at 25 C V Ag AgCl reference V SCE reference 0.241V (2pts each) Circle the correct answer. 1. Which of the following is the strongest reducing agent under standard conditions? a. Sn +4 b. Sn +2 c. S 2 O 3-2 d. H + 2. The E for the reduction of phosphate under basic conditions is V (reaction below). If the ph is increased, which statement is true? PO H 2 O + 1 e -! HPO OH -1 a. Phosphate will be a stronger oxidizing agent. b. Can t predict oxidizing strength from the information given. c. Phosphate oxidizing strength will stay the same. d. Phosphate will be a weaker oxidizing agent. 3. Which of the following is not true about the a glass ph electrode? a. High concentrations of other positive ions in the solution produce ph readings that are too high. b. Highly acidic solutions produce ph readings that are too high. c. Varying junction potentials are the main reason the accuracy of ph readings is limited to two decimal places. d. The response time for ph electrodes can be quite slow in low ionic strength solutions. 4. If you are titrating a mixture of tin(ii) and iron(ii) with a Ce +4 solution, which species will be oxidized first? E for Ce +4 /Ce +3 = 1.47 V, E for Fe +3 /Fe +2 = V, and E for Sn +4 /Sn +2 = 0.15 V. a. Fe +2 b. Sn +2 c. Ce +4 d. can t tell from information given. 5. Ion selective electrodes that do not involve a redox process produce a voltage that follows the equation given below. For a cyanide specific electrode which statement is true? a. E decreases when [CN - ] increases E = const + log [X] n c. The constant decreases when [CN - ] increases. b. E increases when [CN - ] increases. d. The constant increases when [CN - ] increases. (remainder of the exam on the back)

8 On the rest of the exam: SHOW YOUR WORK. NO WORK, NO CREDIT! INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS! USE PAPER PROVIDED -- ONE SIDE OF PAGE! NAME OR INITIALS ON EACH PAGE.! UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS. Table of constants on front side. 6. Permanganate in neutral or basic solution is often used to identify the presence of alkenes, and with a little quantitative analysis to also identify the location of the double bond. The unbalance reaction for a particular butene is: MnO 4-1 (aq) + C 4 H 8 (aq)! CH 3 COO -1 (aq) + MnO 2 (s) a. (20 pts) Balance the reaction with basic conditions. b. (10 pts) Write the standard cell (line) notation for this reaction when it is set up as a battery. c. (20 pts) If E for the butene half reaction is V, what is K eq for this reaction? (The permanganate E is in the table). 7. (15 pts) Mercaptans (RSH) can be titrated with electrically generated Cu +2 in the reaction Cu RSH 6 Cu(SR) 2 (s) + 2H +. Using a copper anode with a current of A it took 120 seconds to completely precipitate all the mercaptan in 50.0 ml of solution. What was the molarity of the RSH in the solution? 8. (25 pts) Enough sodium iodide is dissolved in a saturated solution of lead iodide to make the iodide concentration 4.38 x 10-3 M. A strip of lead metal is inserted into the solution to form a half cell. If the K sp of lead iodide is 7 x 10-9, what is the reduction voltage for this half cell? Don t forget the Honor Pledge