Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods389In an indirect volatilization gravimetric analysis, the change in thesample’s weight is proportional to the amount of analyte. Note that in thefollowing example it is not necessary to apply a conservation of mass torelate the analytical signal to the analyte.Example 8.6A sample of slag from a blast furnace is analyzed for SiO 2 by decomposinga 0.5003-g sample with HCl, leaving a residue with a mass of 0.1414 g.After treating with HF and H 2 SO 4 , and evaporating the volatile SiF 4 , aresidue with a mass of 0.0183 g remains. Determine the %w/w SiO 2 inthe sample.So l u t i o nIn this procedure the difference in the residue’s mass before and after volatilizingSiF 4 gives the mass of SiO 2 in the sample; thus the sample containsand the %w/w SiO 2 is0. 1414 g− 0. 0183 g=0.1231 gSiO 20.1231 gSiO0.5003 g2× 100 = 24. 61%w/wSiO2Practice Exercise 8.7Heating a 0.3317-g mixture of CaC 2 O 4 and MgC 2 O 4 yields a residue of0.1794 g at 600 o C and a residue of 0.1294 g at 1000 o C. Calculate the%w/w CaC 2 O 4 in the sample. You may wish to review your answer toPractice Exercise 8.6 as you consider this problem.Click here to review your answer to this exercise.Finally, in some quantitative applications we can compare the result fora sample to a similar result obtained using a standard.Example 8.7A 26.23-mg sample of MgC 2 O 4 •H 2 O and inert materials is heated to constantweight at 1200 o C, leaving a residue weighing 20.98 mg. A sampleof pure MgC 2 O 4 •H 2 O, when treated in the same fashion, undergoes a69.08% change in its mass. Determine the %w/w MgC 2 O 4 •H 2 O in thesample.So l u t i o nThe change in the sample’s mass is 5.25 mg, which corresponds to