Page 285 - Modern Analytical Chemistry

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268 Modern Analytical Chemistry
Modern Analytical Chemistry
9. Iron can be determined gravimetrically by precipitating as 11. A sample of an impure iron ore is believed to be
Fe(OH) 3 and igniting to Fe 2 O 3 . The sample to be analyzed is approximately 55% w/w Fe. The amount of Fe in the sample
weighed and transferred to a 400-mL beaker where it is is to be determined gravimetrically by isolating it as Fe 2 O 3 .
dissolved in 50 mL of H 2 O and 10 mL of 6 M HCl. Any Fe 2+ How many grams of sample should be taken to ensure that
3+
that is present is oxidized to Fe with 1–2 mL of concentrated approximately 1 g of Fe 2 O 3 will be isolated?
HNO 3 . After boiling to remove the oxides of nitrogen, the 12. The concentration of arsenic in an insecticide can be
solution is diluted to 200 mL, brought to boiling, and determined gravimetrically by precipitating it as
Fe(OH) 3 is precipitated by slowly adding 1:1 NH 3 until the MgNH 4 AsO 4 . The precipitate is ignited and weighed as
odor of NH 3 is detected. The solution is boiled for an Mg 2 As 2 O 7 . Determine the %w/w As 2 O 3 in a 1.627-g sample of
additional minute, and the precipitate is allowed to settle to insecticide that yields 106.5 mg of Mg 2 As 2 O 7 .
the bottom of the beaker. The precipitate is then filtered and
13. After preparing a sample of alum, K 2 SO 4 • Al 2 (SO 4 ) 3 • 24H 2 O,
washed with several portions of hot 1% w/v NH 4 NO 3 a student determined its purity gravimetrically. A 1.2931-g
–
until no Cl is found in the wash water. Finally, the
precipitate is ignited to constant weight at 500–550 °C, sample was dissolved and the aluminum precipitated as
and weighed as Fe 2 O 3 . Al(OH) 3 . The precipitate was collected by filtration, washed,
(a) If the ignition is not carried out under oxidizing and ignited to Al 2 O 3 , yielding 0.1357 g. What is the purity of
conditions (plenty of O 2 present), the final product will the alum preparation?
contain some Fe 3 O 4 . What effect would this have on the 14. To determine the amount of iron in a dietary supplement, a
reported %w/w Fe? (b) The precipitate is washed with a dilute random sample of 15 tablets weighing a total of 20.505 g was
solution of NH 4 NO 3 . Why is NH 4 NO 3 added to the wash ground into a fine powder. A 3.116-g sample of the powdered
water? (c) Why does the procedure call for adding NH 3 until tablets was dissolved and treated to precipitate the iron as
the odor of ammonia is detected? (d) Describe how you might Fe(OH) 3 . The precipitate was collected, rinsed, and ignited to
test the filtrate for Cl . a constant weight as Fe 2 O 3 , yielding 0.355 g. Report the iron
–
content of the dietary supplement as g FeSO 4× 7H 2 O per
10. Sinha and Shome described a gravimetric method for
molybdenum in which it is precipitated with n- tablet.
benzoylphenylhydroxylamine, C 13 H 11 NO 2 , giving a 15. A 1.4639-g sample of limestone was analyzed for Fe, Ca, and
12
precipitate of MoO 2 (C 13 H 10 NO 2 ) 2 . The precipitate is Mg. The iron was determined as Fe 2 O 3, yielding 0.0357 g.
weighed after igniting to MoO 3 . As part of their study they Calcium was isolated as CaSO 4 , yielding a precipitate of
determined the optimum conditions for the analysis. 1.4058 g, and Mg was isolated as 0.0672 g of Mg 2 P 2 O 7 . Report
Samples containing a known amount of Mo were taken the amount of Fe, Ca, and Mg in the limestone sample as
through the procedure while varying the temperature, the %w/w Fe 2 O 3 , %w/w CaO, and %w/w MgO.
amount of precipitant added, and the pH of the solution. 16. The number of ethoxy groups (CH 3 CH 2 O–) in an organic
The solution volume was held constant at 300 mL for all compound can be determined by the following sequence of
experiments. A summary of their results are shown in the reactions
following table
R(OCH 2 CH 3 ) x + xHI ® R(OH) x + xCH 3 CH 2 I
g Mo Temperature g mL 10 M +
taken (°C) precipitant HCl g MoO 3 CH 3 CH 2 I+Ag +H 2 O ® AgI(s)+CH 3 CH 2 OH
A 36.92-mg sample of an organic compound with an
0.0770 30 0.20 0.9 0.0675
approximate molecular weight of 176 was treated in this
0.0770 30 0.30 0.9 0.1014
fashion, yielding 0.1478 g of AgI. How many ethoxy groups
0.0770 30 0.35 0.9 0.1140 are there in each molecule?
0.0770 30 0.42 0.9 0.1155
17. A 516.7-mg sample containing a mixture of K 2SO 4 and
0.0770 30 0.42 0.3 0.1150
(NH 4) 2SO 4 was dissolved in water and treated with BaCl 2,
0.0770 30 0.42 18.0 0.1152 precipitating the SO 4 as BaSO 4. The resulting precipitate was
2–
0.0770 30 0.42 48.0 0.1160 isolated by filtration, rinsed free of impurities, and dried to a
0.0770 30 0.42 75.0 0.1159 constant weight, yielding 863.5 mg of BaSO 4. What is the
0.0770 50 0.42 0.9 0.1156 %w/w K 2 SO 4 in the sample?
0.0770 75 0.42 0.9 0.1158 18. The amount of iron and manganese in an alloy can be
0.0770 80 0.42 0.9 0.1129 determined by precipitating the metals with 8-
hydroxyquinoline, C 9H 7NO. After weighing the mixed
Considering these results, discuss the optimum conditions
precipitate, the precipitate is dissolved and the amount of
for the determination of Mo by this method. Express
8-hydroxyquinoline determined by another method. In a
your results for the precipitant as the minimum
typical analysis, a 127.3-mg sample of an alloy containing
concentration in excess, as %w/v, needed to ensure a
iron, manganese, and other metals was dissolved in acid and
quantitative precipitation.