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Chapter 8 Gravimetric Methods of Analysis 269
treated with appropriate masking agents to prevent an 23. Two methods have been proposed for the analysis of sulfur in
interference from other metals. The iron and manganese were impure samples of pyrite, FeS 2 . Sulfur can be determined in a
precipitated and isolated as Fe(C 9 H 6 NO) 3 and direct analysis by oxidizing it to SO 4 and precipitating as
2–
Mn(C 9 H 6 NO) 2 , yielding a total mass of 867.8 mg. The BaSO 4 . An indirect analysis is also possible if the iron is
amount of 8-hydroxyquinolate in the mixed precipitate was precipitated as Fe(OH) 3 and isolated as Fe 2 O 3 . Which of these
determined to be 5.276 mmol. Calculate the %w/w Fe and methods will provide a more sensitive determination for
%w/w Mn in the alloy. sulfur? What other factors should be considered in deciding
19. A 0.8612-g sample of a mixture consisting of NaBr, NaI, and between these methods?
NaNO 3 was analyzed by adding AgNO 3 to precipitate the Br – 24. A sample of impure pyrite known to be approximately
–
and I , yielding a 1.0186-g mixture of AgBr and AgI. The 90–95% w/w FeS 2 is to be analyzed by oxidizing the sulfur to
precipitate was then heated in a stream of Cl 2 , converting it to SO 4 and precipitating as BaSO 4 . How many grams of the
2–
0.7125 g of AgCl. Calculate the %w/w NaNO 3 in the sample. sample must be taken if a minimum of 1 g of BaSO 4 is
20. The earliest determinations of elemental atomic weights were desired?
accomplished gravimetrically. In determining the atomic 25. A series of samples consisting of any possible combination of
KCl, NaCl, and NH 4 Cl is to be analyzed by adding AgNO 3 to
weight of manganese, a carefully purified sample of MnBr 2
–
weighing 7.16539 g was dissolved and the Br precipitated as precipitate AgCl. What is the minimum volume of 5% w/v
AgBr, yielding 12.53112 g. What is the atomic weight for Mn AgNO 3 necessary to completely precipitate the chloride in any
if the atomic weights for Ag and Br are taken to be 107.868 0.5-g sample?
and 79.904, respectively?
26. When precipitation does not result in a “stoichiometric”
21. While working as a laboratory assistant you prepared 0.4 M
precipitate, a gravimetric analysis is still feasible if the
solutions of AgNO 3 , Pb(NO 3 ) 2 , BaCl 2 , KI, and Na 2 SO 4 .
stoichiometric ratio between analyte and precipitate can be
Unfortunately, you became distracted and forgot to label the
established experimentally. Consider, for example, the
solutions before leaving the laboratory. Realizing your error,
precipitation gravimetric analysis of Pb as PbCrO 4 . (a) For
you labeled the solutions A–E and performed all possible
each gram of Pb in a sample, how many grams of PbCrO 4 are
binary mixings of the five solutions. The following results
expected to form? (b) In a study of this procedure, Grote
were obtained where NR means no reaction was observed, W 14
found that 1.568 g of PbCrO 4 formed per gram of Pb. What
means a white precipitate formed, and Y means a yellow
is the apparent stoichiometry between Pb and PbCrO 4 ?
precipitate formed.
(c) Does failing to account for the actual stoichiometry lead to
a positive or negative determinate error for the analysis?
A B C D E
27. Determine the uncertainty for the gravimetric analysis
A NR Y NR W described in Example 8.1. (a) How does your result compare
with the expected accuracy of 0.1–0.2% for precipitation
B Y W W
gravimetry? (b) What sources of error might account for any
discrepancy between the most probable measurement error
C NR NR
and the expected accuracy?
D W
28. A 38.63-mg sample of potassium ozonide, KO 3 , was heated to
70 °C for 1 h, undergoing a weight loss of 7.10 mg. Write a
Identify solutions A–E. balanced chemical reaction describing this decomposition
22. A solid sample is known to consist of approximately equal reaction. A 29.6-mg sample of impure KO 3 experiences a
amounts of two or more of the following soluble salts. 4.86-mg weight loss when treated under similar condition.
What is the %w/w KO 3 in the sample?
AgNO 3 ZnCl 2 K 2 CO 3 MgSO 4 Ba(C 2 H 3 O 2 ) 2 NH 4 NO 3
29. The water content of an 875.4-mg sample of cheese was
A sample of the solid, sufficient to give at least 0.04 mol of any
determined with a moisture analyzer. What is the %w/w H 2O
single salt, was added to 100 mL of water, yielding a white
in the cheese if the final mass was found to be 545.8 mg?
precipitate and a clear solution. The precipitate was collected
30. Method 8.2 describes a procedure for determining Si in ores
and rinsed with water. When a portion of the precipitate was
and alloys. In this analysis a weight loss of 0.21 g corresponds
placed in dilute HNO 3 , it completely dissolved, leaving a
to 0.1 g of Si. Show that this relationship is correct.
colorless solution. A second portion of the precipitate was
placed in dilute HCl, yielding a precipitate and a clear 31. The iron content of an organometallic compound was
solution. Finally, the filtrate from the original precipitate was determined by treating a 0.4873-g sample with HNO 3 and
treated with excess NH 3, yielding a white precipitate. Indicate heating to volatilize the organic material. After ignition, the
which of the soluble salts must be present in the sample, residue of Fe 2O 3 was found to weigh 0.2091 g. (a) What is the
which must be absent and those for which there is insufficient %w/w Fe in this compound? The carbon and hydrogen in a
information to make this determination. 13 second sample of the compound were determined by a