CHE 226 ANALYTICAL CHEMISTRY Fall 2005

Size: px

Start display at page:

Download "CHE 226 ANALYTICAL CHEMISTRY Fall 2005"

Job Taylor
6 years ago
Views:

1 CHE 226 ANALYTICAL CHEMISTRY EXAM I September 22, 2005 Name WRITE YOUR NAME ON EACH EXAM PAGE NOW. THERE ARE 10 QUESTIONS AND 106 PERCENT TOTAL IN THIS EXAM. Show clearly all work on these pages. Use the proper number of significant figures and the correct units in all final answers. You must show your calculations and/or reasoning, including equations, on a question to obtain any credit; no credit for answers appearing out of the blue. Your work must be understandable at the time it is being graded to obtain any partial credit. You do not have to do the final arithmetic on a question unless you need to have a numerical value for the next part of a question, as long as the answer is expressed in its final form and all algebraic manipulations have been made. Very little will be subtracted for routine arithmetic errors, but all numerical answers must be shown to the proper number of significant figures. Programmable calculators must have all memory erased. A calculator may be used, but not shared with anyone else. Tables of data and other information that may be useful are appended to the back of the exam. Use the backs of the pages as scrap paper. Anything written on the backs of pages is totally irrelevant to the grading process. Unless otherwise stated, assume all solutions are aqueous, density = g/ml; activity coefficients are unity (i.e., activity = concentration); temperature, T = 298 K; K w = x QUESTION 1 / 9 Question 7 /8 QUESTION 2 /12 Question 8 /8 QUESTION 3 /6 Question 9 /5 QUESTION 4 /4 Question 10 /8 QUESTION 5 /17 Question 11 / QUESTION 6 /29 TOTAL /106

2 1. (9 Points) Express the results of the following calculations, as obtained from the readout of a hand calculator, to the proper number of significant figures. That is, round off the numerical answers indicated according to the simple, nominal rules for rounding. (a) = = (b) = = (c) log 10 (0.0230) = = 2. (12 points) A g sample of coal was combusted in a pure oxygen atmosphere. The sulfur dioxide generated was catalytically converted to sulfate, trapped in a reagent solution, and precipitated as insoluble calcium sulfate, CaSO 4 ( g/mol). The calcium sulfate precipitate was filtered, dried, and weighed to be g. If the atomic mass of sulfur is g/mol, calculate the % S by mass in the coal sample. 3. (6 points) List two of the important characteristics of indeterminate or random error. CHE 226 Exam I 2

3 4. (4 Points) Define the term accuracy. 5. (17 Points) Calculate the absolute standard deviation in the calculated result y for the functions shown below. The absolute standard deviation for a measured quantity is shown in parentheses. Show your work. For your final answer, report the calculated result and its absolute standard deviation to the appropriate number of significant figures. (a) (5 Points) y = 1.97(± 0.01) = (± 3) (b) (12 Points) y = 157(± 6) 59(± 3) = (± 1) + 77(± 8) CHE 226 Exam I 3

4 6. (27 Points) Consider the following set of replicate measurements (in µg/g) and then calculate: 65, 50, 62, 69, 63 (a) (3 Points) The median. (b) (3 Points) The range (c) (4 Points) The mean or average. (d) (7 Points) The relative standard deviation, s rel, in percent. (e) (8 Points) The 99% confidence limit of the set of data if the σ for this type of measurement is known to be ± 5 µg/g from a very large number of measurements [s σ]. CHE 226 Exam I 4

5 (f) (4 Points) If the accepted value for the sample analyzed is 65 µg/g, calculate the relative error in the set of data in %. 7. (8 Points) Five replicate values were obtained for the concentration of trace iron in a standard sample of freeze-dried bovine liver: 36, 31, 37, 36, and 40 ppm. Can any of the experimental values be rejected at the 90% confidence level? That is, are there any statistically significant outliers? 8. (8 Points) The maximum upper limit for the pesticide chlordane (now illegal to use in the U.S.) in domestic drinking water is ppb (3 parts per trillion!). If the molar mass of chlordane (C 10 H 6 Cl 8 ) is , convert this upper limit to molar units? CHE 226 Exam I 5

6 9. (5 Points) Very briefly, what is the primary distinction between x and µ and between s and σ in the statistical analysis of experimental data? 10. (8 points) In a Normal or Gaussian distribution, what percentage of all the data would be expected to have values greater than µ + 2σ? [Strong hint: Draw a picture.] CHE 226 Exam I 6

7 Confidence Levels for Various Values of z Confidence Level, % z [ z = (x µ)/σ ] Values of t for Various Levels of Probability Two-Tailed Test (±) Degrees of Factors for the Confidence Interval Freedom, ν 80% 90% 95% 99% 99.9%_ Number of Q crit (Reject if Q calc > Q crit ) Observations, n 90% C.L. 95% C.L. 99% C.L CHE 226 Exam I 7

8 SELECTED CONSTANTS, UNITS, AND CONVERSION FACTORS [The uncertainty in the last digit(s) is shown italicized in parentheses] Atomic mass constant: m u = x kg Avogadro s number: N = (47) x mol -1 Boltzmann constant: k = (24) x J/K Elementary charge: e = (63) x C Faraday constant: F = (39) C/mol Molar gas constant: R = (15) J/K-mol = cal/k-mol = L-atm/K-mol = m 3 /mol at STP Pi: π = Planck s constant: h = (52) x J-s Speed of light (in a vacuum): c = (exact) x 10 8 m/s Stefan-Boltzmann constant: σ = (40) x 10-8 W/m 2 -K 4 Standard acceleration of gravity: g n = (exact) m/s 2 Wein constant: k = (51) x 10-3 m-k Force: 1 N = 1 kg-m/s 2 Joule: 1 J = 1 N-m = 1 kg-m 2 /s 2 = 10 7 ergs = 1 V x 1 C = 1 V-C = (J/C)(C) Power: 1 W = 1 J/s = 1 V x 1 A = 1 V-A = (J/C)(C/s) Electron Volt: 1 ev = (63) x J = x cal Calorie (thermochemical): 1 cal = J [Food calorie = 1 Cal = 1000 cal] Length: 1 km = 1000 m = mi 1 in = 2.54 cm (exactly) Mass: 1 kg = 1000 g 1 pound = g Pressure: (exact) Pa = 1 atm = 760 mm Hg = lb/in Pa = 1 torr = 1 mm Hg 10 5 Pa = 1 bar 1 Pa = 1 N/m 2 Volume: 1 L = 10-3 m 3 = 1000 ml = 1000 cm 3 = quarts Nernst factor: (RT/nF) ln = ( V/n) log 10 at 25 o C nf/rt = n V -1 RT/nF = /n V G o = -nfe o = -RT(ln K eq ) G = -nfe = -RT(ln Q) µ = x + zσ/n 1/2 µ = x + ts/n 1/2 Some Less Common Multiplicative Prefixes: P = peta = T = tera = G = giga = 10 9 n = nano = 10-9 p = pico = f = femto = a = atto = z = zepto = y = yocto = [See for additional information.] CHE 226 Exam I 8

CHE 226 ANALYTICAL CHEMISTRY Spring 2005

CHE 226 ANALYTICAL CHEMISTRY Spring 2005 CHE 226 ANALYTICAL CHEMISTRY EXAM II March 10, 2005 Name WRITE YOUR NAME ON EACH EXAM PAGE NOW. THERE ARE 10 QUESTIONS AND 109 POINTS TOTAL IN THIS EXAM. Show clearly all work on these pages. Use the proper