Analytical Chem istry - DePauw University

500 Analytical Chemistry 2.0Practice Exercise 9.20The purity of a sample of sodium oxalate, Na 2 C 2 O 4 , is determined bytitrating with a standard solution of KMnO 4 . If a 0.5116-g sample requires35.62 mL of 0.0400 M KMnO 4 to reach the titration’s end point,what is the %w/w Na 2 C 2 O 4 in the sample.Click here to review your answer to this exercise.As shown in the following two examples, we can easily extend this approachto an analysis that requires an indirect analysis or a back titration.Example 9.12A 25.00-mL sample of a liquid bleach was diluted to 1000 mL in a volumetricflask. A 25-mL portion of the diluted sample was transferred bypipet into an Erlenmeyer flask containing an excess of KI, reducing theOCl – to Cl – , and producing I 3 – . The liberated I 3 – was determined bytitrating with 0.09892 M Na 2 S 2 O 3 , requiring 8.96 mL to reach the starchindicator end point. Report the %w/v NaOCl in the sample of bleach.The balanced reactions for this analysis are:− − +OCl ( aq ) + 3I ( aq ) + 2H( aq ) →− −( aq) + Cl ( aq) + HO( l)32I−2−2−−I ( aq ) + 2S O ( aq ) → S O ( aq ) + 3I ( aq )3 2 34 6So l u t i o nTo determine the stoichiometry between the analyte, NaOCl, and thetitrant, Na 2 S 2 O 3 , we need to consider both the reaction between OCl –and I – , and the titration of I 3 – with Na 2 S 2 O 3 .First, in reducing OCl – to Cl – , the oxidation state of chlorine changesfrom +1 to –1, requiring two electrons. The oxidation of three I – to formI 3 – releases two electrons as the oxidation state of each iodine changes from–1 in I – to –⅓ in I 3 – . A conservation of electrons, therefore, requires thateach mole of OCl – produces one mole of I 3 – .Second, in the titration reaction, I 3 – is reduced to I – and S 2 O 3 2– is oxidizedto S 4 O 6 2– . Reducing I 3 – to 3I – requires two elections as each iodine changesfrom an oxidation state of –⅓ to –1. In oxidizing S 2 O 32–to S 4 O 6 2– , eachsulfur changes its oxidation state from +2 to +2.5, releasing one electronfor each S 2 O 3 2– . A conservation of electrons, therefore, requires that eachmole of I 3 – reacts with two moles of S 2 O 3 2– .Finally, because each mole of OCl – produces one mole of I 3 – , and eachmole of I 3 – reacts with two moles of S 2 O 3 2– , we know that every mole ofNaOCl in the sample ultimately results in the consumption of two molesof Na 2 S 2 O 3 .The moles of Na 2 S 2 O 3 used in reaching the titration’s end point is( 0. 09892 MNaSO) × ( 0.00896 LNaSO)2 2 3 2 2 3= 88 . 6× 10 −4molNaSO2 2 3which means the sample contains