Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods407Practice Exercise 8.4To find the mass of (NH 4 ) 3 PO 4 •12MoO 3 producing 0.600 g of Pb-MoO 3 , we first use a conservation of mass for molybdenum; thus0.600 gPbMoO31 molMo× ×351.14 gPbMoO31876. 38 g(NH ) PO i12MoO4 3 4 3=12 molMo0.2672 g( NH ) 3PO i12MoO4 4 3Next, to convert this mass of (NH 4 ) 3 PO 4 •12MoO 3 to a mass of Na 3 PO 4 ,we use a conservation of mass on PO 4 3– .0. 2672 g(NH ) 3PO i 12MoO×4 4 31 molPO3-41876.38 g(NH ) PO i12MoO4 3 4 3163.94 gNaPO×molPO= 0.02335 gNaPO3 43- 3 44Finally, we convert this mass of Na 3 PO 4 to the corresponding mass ofsample.0.02335 gNaPO3 4100 gsample× =0. 187 gsample12.5 gNaPO3 4A sample of 0.187 g is sufficient to guarantee 0.600 g of PbMoO 3 . If asample contains more than 12.5% Na 3 PO 4 , then a 0.187-g sample willproduce more than 0.600 g of PbMoO 3 .Click here to return to the chapter.Practice Exercise 8.5To determine which form has the greatest sensitivity, we use a conservationof mass for iron to find the relationship between the precipitate’s massand the mass of iron.1 molFe 71.85 gFeOgFeO = gFe× ×55.85 gFe mol Fe= 1.286×gFegFeO = molFegFe × 1 gFe2 355.85 gFe× 159 70 O2mol Fe.2 3gFeO = molFegFe × 231.55 gFe3 455.85 gFe×O3mol Fe13 4= 1.430×gFe= 1.382×gFe