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What weight of impure NaCl sample must be taken for analysis so that the weight of AgCl precipitate obtained in mg will be equal to the %Cl in the sample? A 6.881 g sample containing magnesium chloride (95.211) and sodium chloride was dissolved in sufficient water to give 500 ml of solution. Analysis for the chloride content of a 50 ml of aliquot resulted in the formation of 0.5923 gram AgCl (143.32). The magnesium in a second 50 ml aliquot was precipitated as MgNH4PO4; on ignition, 0.1796 gram of Mg2P2O7 (222.57) was found. Calculate the % NaCl (58.45) in the sample. A sample of impure pyrite known to be approximately 90–95% w/w FeS2 (119.98) is to be analyzed by oxidizing the sulfur to SO42– and precipitating as BaSO4 (233.38). How many grams of the sample must be taken if a minimum of 1 g of BaSO4 is desired? The calcium in a 200.0-mL sample of a natural water was determined by precipitating the cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO (56.077 g/mol) was 26.7134 g. Calculate the concentration of Ca (40.078 g/mol) in water in units of grams per 100 mL of the water. An iron ore was analyzed by dissolving a 1.1324-g sample in concentrated HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide Fe2O3 · xH2O by the addition of NH3. After filtration and washing, the residue was ignited at a high temperature to give 0.5394 g of pure Fe2O3 (159.69 g/mol). Calculate (a) the % Fe (55.847 g/mol) and (b) the % Fe3O4 (231.54 g/mol) in the sample. Treatment of a 0.2500-g sample of impure potassium chloride with an excess of AgNO3 resulted in the formation of 0.2912 g of AgCl. Calculate the percentage of KCl in the sample. The aluminum in a 1.200-g sample of impure ammonium aluminum sulfate was precipitated with aqueous ammonia as the hydrous Al2O3 · xH2O. The precipitate was filtered and ignited at 1000°C to give anhydrous Al2O3, which weighed 0.2001 g. Express the result of this analysis in terms of (a) % NH4Al(SO4)2.

(b) % Al2O3.

(c) % Al.

What mass of Cu(IO3)2 can be formed from 0.650 g of CuSO4 · 5H2O? What mass of KIO3 is needed to convert the copper in 0.2750 g of CuSO4 · 5H2O to Cu(IO3)2? What mass of AgI can be produced from a 0.512-g sample that assays 20.1% AlI3? Precipitates used in the gravimetric determination of uranium include Na2U2O7 (634.0 g/mol), (UO2)2P2O7 (714.0 g/mol), and V2O5 · 2UO3 (753.9 g/ mol). Which of these weighing forms provides the greatest mass of precipitate from a given quantity of uranium? A 0.8102-g sample of impure Al2(CO3)3 decomposed with HCl; the liberated CO2 was collected on calcium oxide and found to weigh 0.0515 g. Calculate the percentage of aluminum in the sample.

The hydrogen sulfide in a 80.0-g sample of crude petroleum was removed by distillation and uncollected in a solution of CdCl2. The precipitated CdS was then filtered, washed, and ignited to CdSO4. Calculate the percentage of H2S in the sample if 0.125 g of CdSO4 was recovered. A 0.2121-g sample of an organic compound was burned in a stream of oxygen, and the CO2 produced was collected in a solution of barium hydroxide. Calculate the percentage of carbon in the sample if 0.6006 g of BaCO3 was formed. A 7.000-g sample of a pesticide was decomposed with metallic sodium in alcohol, and the liberated chloride ion was precipitated as AgCl. Express the results of this analysis in terms of percent DDT (C14H9Cl5) based on the recovery of 0.2513 g of AgCl. The mercury in a 1.0451-g sample was precipitated with an excess of paraperiodic acid, H5IO6: 5Hg2+ + 2H5IO6 S

Hg5(IO6 )2

+

10H+

The precipitate was filtered, washed free of precipitating agent, dried, and weighed, and 0.5718 g was recovered. Calculate the percentage of Hg2Cl2 in the sample. The iodide in a sample that also contained chloride was converted to iodate by treatment with an excess of bromine: 3H2O + 3Br2 + I-

6Br- +

IO3- + 6H+

The unused bromine was removed by boiling; an excess of barium ion was then added to precipitate the iodate: Ba2+ + 2IO3-

Ba(IO3)2

In the analysis of a 1.59-g sample, 0.0538 g of barium iodate was recovered. Express the results of this analysis as percent potassium iodide.