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34 Modern Analytical Chemistry

12. A 250.0-mL aqueous solution contains 45.1 mg of a pesticide. 18. A series of dilute NaCl solutions is prepared, starting with an
Express the pesticide’s concentration in weight percent, parts initial stock solution of 0.100 M NaCl. Solution A is prepared
per million, and parts per billion. by pipeting 10 mL of the stock solution into a 250-mL
13. A city’s water supply is fluoridated by adding NaF. The volumetric flask and diluting to volume. Solution B is
–
desired concentration of F is 1.6 ppm. How many milligrams prepared by pipeting 25 mL of solution A into a 100-mL
of NaF should be added per gallon of treated water if the volumetric flask and diluting to volume. Solution C is
–
water supply already is 0.2 ppm in F ? prepared by pipeting 20 mL of solution B into a 500-mL
volumetric flask and diluting to volume. What is the molar
14. What is the pH of a solution for which the concentration of concentration of NaCl in solutions A, B, and C?
+
–6
+
H is 6.92 ´10 M? What is the [H ] in a solution whose pH
is 8.923? 19. Calculate the molar concentration of NaCl, to the correct
number of significant figures, if 1.917 g of NaCl is placed in a
15. Using conservation principles, write stoichiometric
beaker and dissolved in 50 mL of water measured with a
relationships for the following
2+ graduated cylinder. This solution is quantitatively transferred
a. The precipitation of Mg as Mg 2 P 2 O 7
to a 250-mL volumetric flask and diluted to volume. Calculate
b. The acid–base reaction between CaCO 3 and HCl in which
the concentration of this second solution to the correct
H 2 CO 3 is formed
+ number of significant figures.
c. The reaction between AgCl and NH 3 to form Ag(NH 3) 2 –
2+
2–
d. The redox reaction between Cr 2O 7 and Fe to form Cr 3+ 20. What is the molar concentration of NO 3 in a solution
and Fe 3+ prepared by mixing 50.0 mL of 0.050 M KNO 3 with 40.0 mL
of 0.075 M NaNO 3? What is pNO 3 for the mixture?
16. Calculate the molarity of a potassium dichromate solution
–
prepared by placing 9.67 g of K 2Cr 2 O 7 in a 100-mL 21. What is the molar concentration of Cl in a solution prepared
volumetric flask, dissolving, and diluting to the calibration by mixing 25.0 mL of 0.025 M NaCl with 35.0 mL of 0.050 M
mark. BaCl 2 ? What is pCl for the mixture?
17. For each of the following, explain how you would prepare 1.0 22. To determine the concentration of ethanol in cognac a 5.00-mL
+
L of a solution that is 0.10 M in K . Repeat for concentrations sample of cognac is diluted to 0.500 L. Analysis of the diluted
2
+
+
of 1.0 ´10 ppm K and 1.0% w/v K . cognac gives an ethanol concentration of 0.0844 M. What is the
a. KCl b. K 2 SO 4 c. K 3Fe(CN) 6 molar concentration of ethanol in the undiluted cognac?
2J SUGGESTED READINGS
Two useful articles providing additional information on topics A useful resource for information on maintaining a useful
covered in this chapter are laboratory notebook is
MacCarthy, P. “A Novel Classification of Concentration Units,” Kanare, H. M. Writing the Laboratory Notebook, American
J. Chem. Educ. 1983, 60, 187–189. Chemical Society: Washington, DC; 1985.
Schwartz, L. M. “Propagation of Significant Figures,” J. Chem.
Educ. 1985, 62, 693–697.
2K REFERENCES

1. American Public Health Association. Standard Methods for the
Analysis of Waters and Wastewaters, 19th ed., Washington, DC. 1995.