Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods447produced by bubbling SO 2 through H 2 O 2 . Because all the sulfur in H 2 SO 4comes from the sulfanilamide, we can use a conservation of mass to determinethe amount of sulfanilamide in the sample.31 molS3. 010× 10− molH SO × ×2 4molH SO2 41 mol CHNOS 168 gC HNOS6 4 2 2 6 4 2 2× . 18molS molC H NOS6 42 2= 0.5062 gC HNOS6 4 2 20.5062 gC HNOS6 4 2 2× 100 = 98.56%w/w CHNOS6 4 2 20.5136 gsamplePractice Exercise 9.7The concentration of NO 2 in air can be determined by passing the samplethrough a solution of H 2 O 2 , which oxidizes NO 2 to HNO 3 , andtitrating the HNO 3 with NaOH. What is the concentration of NO 2 , inmg/L, if a 5.0 L sample of air requires 9.14 mL of 0.01012 M NaOH toreach the methyl red end pointClick here to review your answer to this exercise.Example 9.4The amount of protein in a sample of cheese is determined by a Kjeldahlanalysis for nitrogen. After digesting a 0.9814-g sample of cheese, the nitrogenis oxidized to NH 4 + , converted to NH 3 with NaOH, and distilledinto a collection flask containing 50.00 mL of 0.1047 M HCl. The excessHCl is back titrated with 0.1183 M NaOH, requiring 22.84 mL to reachthe bromothymol blue end point. Report the %w/w protein in the cheeseassuming that there are 6.38 grams of protein for every gram of nitrogenin most dairy products.For a back titration we must consider twoacid–base reactions. Again, the calculationsare straightforward.So l u t i o nThe HCl in the collection flask reacts with two bases+ −HCl( aq) + NH ( aq) → NH ( aq) + Cl ( aq)3−−HCl( aq) + OH ( aq) → H O() l + Cl ( aq )The collection flask originally contains0. 1047 MHCl× 0. 05000 LHCl = 5.235× 10 −3molHClof which24