Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods4058H(d) What is the 95% confidence interval for the concentration of thioureain this sample assuming one replicate? See the appendices forstatistical tables.Solutions to Practice ExercisesPractice Exercise 8.1The solubility reaction for CaC 2 O 4 is+ 2−CaC O() s Ca ( aq) + CO ( aq ) 2 2 4 2 4pH2–C 2O 4--pK a2= 4.266HC 2O 4To minimize solubility, the pH needs to be basic enough that oxalate,C 2 O 4 2– , does not react to form HC 2 O 4 – or H 2 C 2 O 4 . The ladder diagramfor oxalic acid, including approximate buffer ranges, is shown in Figure8.17. Maintaining a pH greater than 5.3 ensures that C 2 O 4 2– is the onlyimportant form of oxalic acid in solution, minimizing the solubility ofCaC 2 O 4 .pK a1= 1.252H 2C 2O 4Figure 8.17 pH ladder diagramfor oxalic acid, H 2 C 2 O 4 .Click here to return to the chapter.Practice Exercise 8.2A conservation of mass requires that all the zinc in the alloy is found inthe final product, Zn 2 P 2 O 7 . We know that there are 2 moles of Zn permole of Zn 2 P 2 O 7 ; thus0.1163 gZnPO2 2 72 molZn× ×304.72 gZnPO2 2 76539. gZn= 4.991× 10 −2gZnmolZnThis is the mass of Zn in 25% of the sample (a 25.00 mL portion of the100.0 mL total volume). The %w/w Zn, therefore, isFor copper, we find that4.991× 10 −2gZn× 4 × 100 = 27 . 22 % w/wZn0.7336 gsample1mol Cu0.1163 gCuSCN× ×121.64 gCuSCN63.55 gCumolCu= 0.12490.1249 gCu × 4 × 100 = 67 . 88 % w/w Cu0.7336 gsampleClick here to return to the chapter.gCu