Analytical Chem istry - DePauw University

136 Analytical Chemistry 2.011. Which of the following is the best way to dispense 100.0 mL of a reagent:(a) use a 50‐mL pipet twice; (b) use a 25-mL pipet four times;or (c) use a 10-mL pipet ten times?12. You can dilute a solution by a factor of 200 using readily available pipets(1-mL to 100-mL) and volumetric flasks (10-mL to 1000-mL) in eitherone step, two steps, or three steps. Limiting yourself to the glasswarein Table 4.2, determine the proper combination of glassware to accomplisheach dilution, and rank them in order of their most probableuncertainties.13. Explain why changing all values in a data set by a constant amount willchange X but will have no effect on s.Many of the problems that follow require accessto statistical tables. For your convenience,here are hyperlinks to the appendices containingthese tables.Appendix 3: Single-Sided Normal DistributionAppendix 4: Critical Values for the t-TestAppendix 5: Critical Values for the F-TestAppendix 6: Critical Values for Dixon’s Q-TestAppendix 7: Critical Values for Grubb’s Test14. Obtain a sample of a metal from your instructor and determine itsdensity by one or both of the following methods:Method A: Determine the sample’s mass with a balance. Calculate thesample’s volume using appropriate linear dimensions.Method B: Determine the sample’s mass with a balance. Calculate thesample’s volume by measuring the amount of water that it displaces.This can be done by adding water to a graduated cylinder, reading thevolume, adding the sample, and reading the new volume. The differencein volumes is equal to the sample’s volume.Determine the density at least 5 times. (a) Report the mean, the standarddeviation, and the 95% confidence interval for your results. (b)Find the accepted value for the metal’s density and determine the absoluteand relative error for your determination of the metal’s density.(c) Use a propagation of uncertainty to determine the uncertainty foryour method of analysis. Is the result of this calculation consistent withyour experimental results? If not, suggest some possible reasons for thisdisagreement.15. How many carbon atoms must a molecule have if the mean numberof 13 C atoms per molecule is 1.00? What percentage of such moleculeswill have no atoms of 13 C?16. In Example 4.10 we determined the probability that a molecule ofcholesterol, C 27 H 44 O, had no atoms of 13 C. (a) Calculate the probabilitythat a molecule of cholesterol, has 1 atom of 13 C. (b) What is theprobability that a molecule of cholesterol will have two or more atomsof 13 C?17. Berglund and Wichardt investigated the quantitative determinationof Cr in high-alloy steels using a potentiometric titration of Cr(VI) 19 .19 Berglund, B.; Wichardt, C. Anal. Chim. Acta 1990, 236, 399–410.