Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods3790.4320 gHgCl222 molHg× ×472.09 gHgCl1 mol HgClmolHg222= 18302 . × 10 −3molHgCl2Next, we use the conservation of electrons to find the mass of Na 3 PO 3 .−−31 mol e 1mol Na PO3 318302 . × 10 molHgCl × ××2molHgCl 2mol e −147.94 gNaPO23molNaPOFinally, we calculate the %w/w Na 3 PO 3 in the sample.0.13538 gNaPO0.1392 gsample33333= 0.13538× 100 = 97. 26%w/wNaPO3 3gNaPO33As you become comfortable with usingconservation principles, you will see opportunitiesfor further simplifying problems.For example, a conservation of electronsrequires that the electrons released byNa 3 PO 3 end up in the product, Hg 2 Cl 2 ,yielding the following stoichiometric conversionfactor:2mole −molHgCl2 2This conversion factor provides a directlink between the mass of Hg 2 Cl 2 and themass of Na 3 PO 3 .Practice Exercise 8.4One approach for determining phosphate, PO 4 3– , is to precipitate it asammonium phosphomolybdate, (NH 4 ) 3 PO 4 •12MoO 3 . After isolatingthe precipitate by filtration, it is dissolved in acid and the molybdate precipitatedand weighed as PbMoO 3 . Suppose we know that our samplescontain at least 12.5% Na 3 PO 4 and we need to recover a minimum of0.600 g of PbMoO 3 ? What is the minimum amount of sample neededfor each analysis?Click here to review your answer to this exercise.8B.2 Qualitative ApplicationsA precipitation reaction is a useful method for identifying inorganic andorganic analytes. Because a qualitative analysis does not require quantitativemeasurements, the analytical signal is simply the observation that aprecipitate has formed. Although qualitative applications of precipitationgravimetry have been largely replaced by spectroscopic methods of analysis,they continue to find application in spot testing for the presence of specificanalytes. 9Any of the precipitants listed in Table 8.1,Table 8.3, and Table 8.4 can be used for aqualitative analysis.8B.3 Evaluating Precipitation GravimetrySc a l e o f Op e r a t i o nThe scale of operation for precipitation gravimetry is limited by the sensitivityof the balance and the availability of sample. To achieve an accuracy9 Jungreis, E. Spot Test Analysis; 2nd Ed., Wiley: New York, 1997.