D. Harvey - Modern Analytical Chemistry (794078), страница 13
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Other units, such as power,are defined in terms of these base units. When reporting measurements, we must be careful to include only those digits that are significant and to maintain the uncertainty implied by these significant figures when transforming measurements into results.The relative amount of a constituent in a sample is expressed asits concentration. There are many ways to express concentration,the most common of which are molarity, weight percent, volumepercent, weight-to-volume percent, parts per million, and parts perbillion.
Concentrations also can be expressed using p-functions.Stoichiometric relationships and calculations are important inmany quantitative analyses. The stoichiometry between the reactants and products of a chemical reaction is given by the coefficients of a balanced chemical reaction. When it is inconvenient tobalance reactions, conservation principles can be used to establishthe stoichiometric relationships.Balances, volumetric flasks, pipets, and ovens are standardpieces of laboratory instrumentation and equipment that areroutinely used in almost all analytical work. You should be familiar with the proper use of this equipment.
You also shouldbe familiar with how to prepare a stock solution of known concentration, and how to prepare a dilute solution from a stocksolution.2I PROBLEMS1. Indicate how many significant figures are in each of thefollowing numbers.a. 903b. 0.903c. 1.0903d. 0.0903e. 0.09030f. 9.03 × 1022. Round each of the following to three significant figures.a. 0.89377b. 0.89328c. 0.89350d. 0.8997e. 0.089073. Round each of the following to the stated number ofsignificant figures.a. The atomic weight of carbon to four significant figuresb.
The atomic weight of oxygen to three significant figuresc. Avogadro’s number to four significant figuresd. Faraday’s constant to three significant figures4. Report results for the following calculations to the correctnumber of significant figures.a. 4.591 + 0.2309 + 67.1 =b. 313 – 273.15 =c. 712 × 8.6 =d. 1.43/0.026 =e. (8.314 × 298)/96485 =f.
log(6.53 × 10–5) =g. 10–7.14 =h. (6.51 × 10–5) (8.14 × 10–9) =5. A 12.1374-g sample of an ore containing Ni and Co wascarried through Fresenius’ analytical scheme shown in Figure1.1. At point A the combined mass of Ni and Co was found tobe 0.2306 g, and at point B the mass of Co was found to be0.0813 g. Report the weight percent Ni in the ore to thecorrect number of significant figures.6.
Hillebrand and Lundell’s analytical scheme (see Figure 1.2)for the analysis of Ni in ores involves precipitating Ni2+ usingdimethylgloxime. The formula for the precipitate isNi(C4H7N2O2)2. Calculate the precipitate’s formula weight tothe correct number of significant figures.7. An analyst wishes to add 256 mg of Cl– to a reaction mixture.How many milliliters of 0.217 M BaCl2 should be added?8.
A solution of 0.10 M SO42– is available. What is the normalityof this solution when used in the following reactions?a. Pb2+(aq) + SO42–(aq) PbSO4(s)b. HCl(aq) + SO42–(aq)HSO4–(aq) + Cl–(aq)c. SO42– + 4H3O+(aq) + 2e–H2SO3(aq) + 5H2O(l)ttt9. The concentration of lead in an industrial waste stream is 0.28ppm. What is its molar concentration?10. Commercially available concentrated hydrochloric acid is37.0% w/w HCl. Its density is 1.18 g/mL. Using thisinformation calculate (a) the molarity of concentrated HCl,and (b) the mass and volume (in milliliters) of solutioncontaining 0.315 mol of HCl.11.
The density of concentrated ammonia, which is 28.0% w/w NH3,is 0.899 g/mL. What volume of this reagent should be dilutedto 1.0 × 103 mL to make a solution that is 0.036 M in NH3?1400-CH02 9/8/99 3:49 PM Page 3434Modern Analytical Chemistry12. A 250.0-mL aqueous solution contains 45.1 µg of a pesticide.Express the pesticide’s concentration in weight percent, partsper million, and parts per billion.13. A city’s water supply is fluoridated by adding NaF.
Thedesired concentration of F– is 1.6 ppm. How many milligramsof NaF should be added per gallon of treated water if thewater supply already is 0.2 ppm in F–?14. What is the pH of a solution for which the concentration ofH+ is 6.92 × 10–6 M? What is the [H+] in a solution whose pHis 8.923?15. Using conservation principles, write stoichiometricrelationships for the followinga. The precipitation of Mg2+ as Mg2P2O7b. The acid–base reaction between CaCO3 and HCl in whichH2CO3 is formedc. The reaction between AgCl and NH3 to form Ag(NH3)2+d.
The redox reaction between Cr2O72– and Fe2+ to form Cr3+and Fe3+18. A series of dilute NaCl solutions is prepared, starting with aninitial stock solution of 0.100 M NaCl. Solution A is preparedby pipeting 10 mL of the stock solution into a 250-mLvolumetric flask and diluting to volume. Solution B isprepared by pipeting 25 mL of solution A into a 100-mLvolumetric flask and diluting to volume. Solution C isprepared by pipeting 20 mL of solution B into a 500-mLvolumetric flask and diluting to volume. What is the molarconcentration of NaCl in solutions A, B, and C?19.
Calculate the molar concentration of NaCl, to the correctnumber of significant figures, if 1.917 g of NaCl is placed in abeaker and dissolved in 50 mL of water measured with agraduated cylinder. This solution is quantitatively transferredto a 250-mL volumetric flask and diluted to volume. Calculatethe concentration of this second solution to the correctnumber of significant figures.20. What is the molar concentration of NO3– in a solutionprepared by mixing 50.0 mL of 0.050 M KNO3 with 40.0 mLof 0.075 M NaNO3? What is pNO3 for the mixture?16. Calculate the molarity of a potassium dichromate solutionprepared by placing 9.67 g of K2Cr2O7 in a 100-mLvolumetric flask, dissolving, and diluting to the calibrationmark.21.
What is the molar concentration of Cl– in a solution preparedby mixing 25.0 mL of 0.025 M NaCl with 35.0 mL of 0.050 MBaCl2? What is pCl for the mixture?17. For each of the following, explain how you would prepare 1.0L of a solution that is 0.10 M in K+. Repeat for concentrationsof 1.0 × 102 ppm K+ and 1.0% w/v K+.a. KClb. K2SO4c. K3Fe(CN)622.
To determine the concentration of ethanol in cognac a 5.00-mLsample of cognac is diluted to 0.500 L. Analysis of the dilutedcognac gives an ethanol concentration of 0.0844 M. What is themolar concentration of ethanol in the undiluted cognac?2J SUGGESTED READINGSTwo useful articles providing additional information on topicscovered in this chapter areMacCarthy, P. “A Novel Classification of Concentration Units,”J.
Chem. Educ. 1983, 60, 187–189.Schwartz, L. M. “Propagation of Significant Figures,” J. Chem.Educ. 1985, 62, 693–697.2K REFERENCES1. American Public Health Association. Standard Methods for theAnalysis of Waters and Wastewaters, 19th ed., Washington, DC. 1995.A useful resource for information on maintaining a usefullaboratory notebook isKanare, H. M. Writing the Laboratory Notebook, AmericanChemical Society: Washington, DC; 1985.1400-CH03 9/8/99 3:51 PM Page 353ChapterThe Language ofAnalytical ChemistryAnalytical chemists converse using terminology that conveysspecific meaning to other analytical chemists. To discuss and learnanalytical chemistry you must first understand its language.
You areprobably already familiar with some analytical terms, such as“accuracy” and “precision,” but you may not have placed them in theirappropriate analytical context. Other terms, such as “analyte” and“matrix,” may be less familiar. This chapter introduces many importantterms routinely used by analytical chemists. Becoming comfortablewith these terms will make the material in the chapters that followeasier to read and understand.351400-CH03 9/8/99 3:51 PM Page 3636Modern Analytical Chemistry3A Analysis, Determination, and MeasurementanalysisA process that provides chemical orphysical information about theconstituents in the sample or the sampleitself.analytesThe constituents of interest in a sample.matrixAll other constituents in a sample exceptfor the analytes.determinationAn analysis of a sample to find theidentity, concentration, or properties ofthe analyte.measurementAn experimental determination of ananalyte’s chemical or physical properties.The first important distinction we will make is among the terms “analysis,” “determination,” and “measurement.” An analysis provides chemical or physical information about a sample.
The components of interest in the sample are called analytes, and the remainder of the sample is the matrix. In an analysis we determinethe identity, concentration, or properties of the analytes. To make this determination we measure one or more of the analyte’s chemical or physical properties.An example helps clarify the differences among an analysis, a determination,and a measurement. In 1974, the federal government enacted the Safe DrinkingWater Act to ensure the safety of public drinking water supplies.
To comply withthis act municipalities regularly monitor their drinking water supply for potentiallyharmful substances. One such substance is coliform bacteria. Municipal water departments collect and analyze samples from their water supply. To determine theconcentration of coliform bacteria, a portion of water is passed through a membrane filter.
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