D. Harvey - Modern Analytical Chemistry (794078), страница 32
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The resulting solution wasquantitatively transferred to a 100-mL volumetric flaskand diluted to volume with distilled water. A 10-mLaliquot of the solution was pipeted into a 500-mL volumetricflask and diluted to volume. (a) Express the concentrationof Mn in parts per million, and estimate uncertainty by apropagation of uncertainty calculation. (b) Would theuncertainty in the solution’s concentration be improvedby using a pipet to measure the HNO3, instead of agraduated cylinder?7. Hydroscopic materials often are measured by the technique ofweighing by difference. In this technique the material isplaced in a sealed container and weighed.
A portion of thematerial is removed, and the container and the remainingmaterial are reweighed. The difference between the twomasses gives the amount of material that was sampled. Asolution of a hydroscopic material with a gram formulaweight of 121.34 (±0.01) was prepared in the followingmanner. A sample of the compound and its container has amass of 23.5811 g.
A portion of the compound wastransferred to a 100-mL volumetric flask and diluted tovolume. The mass of the compound and container after thetransfer is 22.1559 g. Calculate the molarity of the solution,and estimate its uncertainty by a propagation of uncertaintycalculation.13.
Obtain a sample of a metal from your instructor, anddetermine its density by one or both of the followingmethods:Method A: Obtain the sample’s mass with a balance.Calculate the sample’s volume using appropriate lineardimensions.Method B: Obtain the sample’s mass with a balance.Calculate the sample’s volume by measuring the amount ofwater that it displaces. This can be done by adding water to agraduated cylinder, reading the volume, adding the object,and reading the new volume. The difference in volumes isequal to the object’s volume.Determine the density at least five times.
(a) Report the mean,the standard deviation, and the 95% confidence interval foryour results. (b) Find the accepted value for the density ofyour metal, and determine the absolute and relative error foryour experimentally determined density. (c) Use thepropagation of uncertainty to determine the uncertainty foryour chosen method. Are the results of this calculationconsistent with your experimental results? If not, suggestsome possible reasons for this disagreement.14. How many carbon atoms must a molecule have if the meannumber of 13C atoms per molecule is 1.00? What percent ofsuch molecules will have no atoms of 13C?15.
In Example 4.10 we determined the probability that amolecule of cholesterol, C27H44O, had no atoms of 13C.(a) Calculate the probability that a molecule of cholesterol,has one atom of 13C. (b) What is the probability that amolecule of cholesterol will have two or more atoms of 13C?16. Berglund and Wichart investigated the quantitativedetermination of Cr in high-alloy steels by a potentiometrictitration of Cr6+.21 Before titrating the steel was dissolved inacid and the chromium oxidized to Cr6+ by peroxydisulfate.Following are their results (%w/w Cr) for the analysis of asingle reference steel.8. Show by a propagation of uncertainty calculation that thestandard error of the mean for n determinations is given as—s / √ n.16.96816.88716.92216.97716.84016.85716.88316.7289.
What is the smallest mass that can be measured on ananalytical balance with a tolerance of ±0.1 mg, such that therelative error is less than 0.1%?Calculate the mean, the standard deviation, and the 95%confidence interval about the mean. What does thisconfidence interval mean?10.
Which of the following is the best way to dispense 100.0 mLof a reagent: (a) use a 50-mL pipet twice; (b) use a 25-mLpipet four times; or (c) use a 10-mL pipet ten times?17. Ketkar and co-workers developed a new analytical methodfor measuring trace levels of atmospheric gases.22 Theanalysis of a sample containing 40.0 parts per thousand (ppt)2-chloroethylsulfide yielded the following results11. A solution can be diluted by a factor of 200 using readilyavailable pipets (1-mL to 100-mL) and volumetric flasks(10-mL to 1000-mL) in either one, two, or three steps.Limiting yourself to glassware listed in Table 4.2, determinethe proper combination of glassware to accomplish eachdilution, and rank them in order of their most probableuncertainties.12.
Explain why changing all values in a data set by a constant–amount will change X but will have no effect on s.43.334.831.937.834.431.942.133.635.3(a)Determine whether there is a significant difference betweenthe experimental mean and the expected value at α = 0.05.(b) As part of this study a reagent blank was analyzed 12times, giving a mean of 0.16 ppt and a standard deviation of1.20 ppt. What are the IUPAC detection limit, the limit ofidentification, and limit of quantitation for this methodassuming α = 0.05?1400-CH04 9/8/99 3:55 PM Page 100100Modern Analytical Chemistry18.
To test a spectrophotometer for its accuracy, a solution of60.06 ppm K2Cr2O7 in 5.0 mM H2SO4 is prepared andanalyzed. This solution has a known absorbance of 0.640 at350.0 nm in a 1.0-cm cell when using 5.0 mM H2SO4 as areagent blank. Several aliquots of the solution are analyzedwith the following results0.6390.6380.6400.6390.6400.6390.638Determine whether there is a significant difference betweenthe experimental mean and the expected value at α = 0.01.19.
Monna and co-workers studied the use of radioactive isotopesas a means of dating sediments collected from the bottom oflakes and estuaries.23 To verify this method they analyzed a208Po standard known to have an activity of 77.5 decays/min,obtaining the following results77.0978.0375.3774.9672.4277.5476.8476.0977.8481.1276.6975.75Determine whether there is a significant difference betweenthe mean and the expected value at α = 0.05.20. A 2.6540-g sample of an iron ore known to contain 53.51%w/w Fe is dissolved in a small portion of concentrated HCland diluted to volume in a 250-mL volumetric flask. Aspectrophotometric method is used to determine theconcentration of Fe in this solution, yielding results of 5840,5770, 5650, and 5660 ppm. Determine whether there is asignificant difference between the experimental mean and theexpected value at α = 0.05.21. Horvat and colleagues investigated the application of atomicabsorption spectroscopy to the analysis of Hg in coal fly ash.24Of particular interest was the development of an appropriateprocedure for digesting the samples in order to release the Hgfor analysis.
As part of their study they tested several reagents fordigesting samples. Results obtained with HNO3 and with a 1 + 3mixture of HNO3 and HCl are shown here. All concentrationsare given as nanograms of Hg per gram of sample.HNO3:1 + 3 HNO3–HCl:161159165145160140167147166143156Determine whether there is a significant difference betweenthese methods at α = 0.05.22. Lord Rayleigh, John William Strutt (1842–1919) was one ofthe most well-known scientists of the late nineteenth andearly twentieth centuries, publishing over 440 papers andreceiving the Nobel Prize in chemistry in 1904 for thediscovery of argon. An important turning point in thediscovery of Ar was Rayleigh’s experimental measurementsof the density of N2.
Rayleigh approached this experiment intwo ways: first by taking atmospheric air and removing anyO2 and H2 that was present; and second, by chemicallyproducing N2 by decomposing nitrogen-containingcompounds (NO, N2O, and NH4NO3) and again removingany O2 and H2. His results for the density of N2, published inProc. Roy. Soc. 1894, LV, 340 (publication 210), follow (allvalues are for grams of gas at equivalent volume, pressure,and temperature).AtmosphericOrigin:ChemicalOrigin:2.310172.310242.301432.298692.309862.310102.298902.299402.310102.310282.298162.298492.310012.301822.29889Explain why these data led Rayleigh to look for and discover Ar.23.
Gács and Ferraroli reported a new method for monitoring theconcentration of SO2 in air.25 They compared their methodwith the standard method by sampling and analyzing urbanair from a single location. Air samples were collected bydrawing air through a collection solution for 6 min. Followingis a summary of their results with SO2 concentrationsreported in microliters per cubic meter.Standardmethod: 21.62Newmethod: 21.5422.2024.2723.5424.2523.09 21.0220.5122.3121.3024.6225.72 21.54Using an appropriate statistical test, determine whether thereis any significant difference between the standard and newmethods at α = 0.05.24.
The accuracy of a spectrophotometer can be checkedby measuring absorbances for a series of standarddichromate solutions that can be obtained in sealedcuvettes from the National Institute of Standards andTechnology. Absorbances are measured at 257 nm andcompared with the accepted values. The resultsobtained when testing a newly purchasedspectrophotometer are shown here. Determine ifthe tested spectrophotometer is accurate at α = 0.05.Standard:Measuredabsorbance:Acceptedabsorbance:123450.2872 0.57730.86741.16231.45590.2871 0.57600.86771.16081.456525.
Maskarinec and associates investigated the stability of volatileorganics in environmental water samples.26 Of particularinterest was establishing proper conditions for maintainingthe sample’s integrity between its collection and analysis. Twopreservatives were investigated (ascorbic acid and sodiumbisulfate), and maximum holding times were determined fora number of volatile organics and water matrices. Results (indays) for surface waters follow.methylene chloridecarbon disulfidetrichloroethanebenzene1,1,2-trichloroethane1,1,2,2-tetrachloroethanetetrachloroethenetoluenechlorobenzeneAscorbicacidSodiumbisulfate7723526257334132366254514253856394861400-CH04 9/8/99 3:55 PM Page 101101Chapter 4 Evaluating Analytical DataDetermine whether there is a significant difference in theeffectiveness of the two preservatives at α = 0.10.26.
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