Page 50 - Modern Analytical Chemistry

1400-CH02 9/8/99 3:49 PM Page 33

Chapter 2 Basic Tools of Analytical Chemistry 33

normality (p. 16) quantitative transfer (p. 30) volume percent (p. 18)
parts per billion (p. 18) scientific notation (p. 12) volumetric flask (p. 26)
parts per million (p. 18) significant figures (p. 13) weight percent (p. 18)
p-function (p. 19) SI units (p. 12) weight-to-volume percent (p. 18)
pipet (p. 27) stock solution (p. 30)

2H SUMMARY

There are a few basic numerical and experimental tools with Stoichiometric relationships and calculations are important in
which you must be familiar. Fundamental measurements in ana- many quantitative analyses. The stoichiometry between the reac-
lytical chemistry, such as mass and volume, use base SI units, such tants and products of a chemical reaction is given by the coeffi-
as the kilogram (kg) and the liter (L). Other units, such as power, cients of a balanced chemical reaction. When it is inconvenient to
are defined in terms of these base units. When reporting measure- balance reactions, conservation principles can be used to establish
ments, we must be careful to include only those digits that are sig- the stoichiometric relationships.
nificant and to maintain the uncertainty implied by these signifi- Balances, volumetric flasks, pipets, and ovens are standard
cant figures when transforming measurements into results. pieces of laboratory instrumentation and equipment that are
The relative amount of a constituent in a sample is expressed as routinely used in almost all analytical work. You should be fa-
its concentration. There are many ways to express concentration, miliar with the proper use of this equipment. You also should
the most common of which are molarity, weight percent, volume be familiar with how to prepare a stock solution of known con-
percent, weight-to-volume percent, parts per million, and parts per centration, and how to prepare a dilute solution from a stock
billion. Concentrations also can be expressed using p-functions. solution.

2I PROBLEMS

1. Indicate how many significant figures are in each of the be 0.2306 g, and at point B the mass of Co was found to be
following numbers. 0.0813 g. Report the weight percent Ni in the ore to the
a. 903 b. 0.903 c. 1.0903 correct number of significant figures.
d. 0.0903 e. 0.09030 f. 9.03 ´10 2 6. Hillebrand and Lundell’s analytical scheme (see Figure 1.2)
2+
2. Round each of the following to three significant figures. for the analysis of Ni in ores involves precipitating Ni using
a. 0.89377 b. 0.89328 c. 0.89350 dimethylgloxime. The formula for the precipitate is
d. 0.8997 e. 0.08907 Ni(C 4H 7N 2O 2 ) 2. Calculate the precipitate’s formula weight to
the correct number of significant figures.
3. Round each of the following to the stated number of
–
significant figures. 7. An analyst wishes to add 256 mg of Cl to a reaction mixture.
a. The atomic weight of carbon to four significant figures How many milliliters of 0.217 M BaCl 2 should be added?
b. The atomic weight of oxygen to three significant figures 8. A solution of 0.10 M SO 4 is available. What is the normality
2–
c. Avogadro’s number to four significant figures of this solution when used in the following reactions?
d. Faraday’s constant to three significant figures a. Pb (aq) +SO 4 (aq) t PbSO 4(s)
2+
2–
2–
–
–
4. Report results for the following calculations to the correct b. HCl(aq) +SO 4 (aq) t HSO 4 (aq) +Cl (aq)
–
+
2–
number of significant figures. c. SO 4 +4H 3O (aq) +2e t H 2 SO 3(aq) +5H 2O(l)
a. 4.591 +0.2309 +67.1 = 9. The concentration of lead in an industrial waste stream is 0.28
b. 313 – 273.15 = ppm. What is its molar concentration?
c. 712 ´8.6 =
10. Commercially available concentrated hydrochloric acid is
d. 1.43/0.026 =
37.0% w/w HCl. Its density is 1.18 g/mL. Using this
e. (8.314 ´298)/96485 =
–5
f. log(6.53 ´10 ) = information calculate (a) the molarity of concentrated HCl,
g. 10 –7.14 = and (b) the mass and volume (in milliliters) of solution
–9
–5
h. (6.51 ´10 ) (8.14 ´10 ) = containing 0.315 mol of HCl.
11. The density of concentrated ammonia, which is 28.0% w/w NH 3 ,
5. A 12.1374-g sample of an ore containing Ni and Co was is 0.899 g/mL. What volume of this reagent should be diluted
carried through Fresenius’ analytical scheme shown in Figure to 1.0 ´10 mL to make a solution that is 0.036 M in NH 3 ?
3
1.1. At point A the combined mass of Ni and Co was found to