Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods39913. The concentration of arsenic in an insecticide is determined gravimetricallyby precipitating MgNH 4 AsO 4 and isolating Mg 2 As 2 O 7 . Determinethe %w/w As 2 O 3 in a 1.627-g sample of insecticide if it yields106.5 mg of Mg 2 As 2 O 7 .14. After preparing a sample of alum, K 2 SO 4 •Al 2 (SO 4 ) 3 •24H 2 O, a studentdetermined its purity by dissolving a 1.2391-g sample and precipitatingthe aluminum as Al(OH) 3 . After filtering, rinsing, and igniting, 0.1357g of Al 2 O 3 is obtained. What is the purity of the alum preparation?15. To determine the amount of iron in a dietary supplement, a randomsample of 15 tablets weighing a total of 20.505 g was ground into a finepowder. A 3.116-g sample was dissolved and treated to precipitate theiron as Fe(OH) 3 . The precipitate was collected, rinsed, and ignited toa constant weight as Fe 2 O 3 , yielding 0.355 g. Report the iron contentof the dietary supplement as g FeSO 4 •7H 2 O per tablet.16. A 1.4639-g sample of limestone was analyzed for Fe, Ca, and Mg. Theiron was determined as Fe 2 O 3 yielding 0.0357 g. Calcium was isolatedas CaSO 4 , yielding a precipitate of 1.4058 g, and Mg was isolated as0.0672 g of Mg 2 As 2 O 7 . Report the amount of Fe, Ca, and Mg in thelimestone sample as %w/w Fe 2 O 3 , %w/w CaO, and %w/w MgO.17. The number of ethoxy groups (CH 3 CH 2 O–) in an organic compoundis determined by the following two reactions.R(CH CH ) + xHI→ R(OH) + xCH CH I2 3 xx3 2CH CH I+ Ag + H O→ AgI()s + CH CH OH3 2+ 2 3 2A 36.92-mg sample of an organic compound with an approximate molecularweight of 176 was treated in this fashion, yielding 0.1478 g ofAgI. How many ethoxy groups are there in each molecule of the compound?18. A 516.7-mg sample containing a mixture of K 2 SO 4 and (NH 4 ) 2 SO 4was dissolved in water and treated with BaCl 2 , precipitating the SO 42–as BaSO 4 . The resulting precipitate was isolated by filtration, rinsedfree of impurities, and dried to a constant weight, yielding 863.5 mg ofBaSO 4 . What is the %w/w K 2 SO 4 in the sample?19. The amount of iron and manganese in an alloy can be determinedby precipitating the metals with 8-hydroxyquinoline, C 9 H 7 NO. Afterweighing the mixed precipitate, the precipitate is dissolved and theamount of 8-hydroxyquinoline determined by another method. In atypical analysis a 127.3-mg sample of an alloy containing iron, manga-