Organic Chemistry Second Hour Exam Key with Formula Sheet | CEM 262, Exams of Organic Chemistry

Download Organic Chemistry Second Hour Exam Key with Formula Sheet | CEM 262 and more Exams Organic Chemistry in PDF only on Docsity! Chemistry 262 Second Hour Exam KEY Spring 2011 Page 1 1a. (5 points) What is the ionic strength, m , of a 0.200 M KIO3 solution at 25 °C? ( )( ) ( )( )m é ù= - + =ê úë û 2 21 2 .0200 1 .0200 1 0.200 M 1b. (10 points) You then prepare a saturated solution of Ba(IO3)2 in this 0.200 M KIO3 solution at 25 °C. What is the 2Ba +é ùê úë û at saturation? Use activities for this calculation. The spK for Ba(IO3)2 is -´ 91.57 10 . Activity coefficients are: g + =2 0.38Ba and g - =3 0.78IO . The molecular weight of Ba(IO3)2 is 391.136 g/mol. ( )( ) ( ) ( ) ( ) 2+ 3 2+ 2+ ( )3 2 3 2 22 2+ 2+ 3IO BaBa IO3 22+ 3 2 Ba IO3 9 2 2 29 9 2 7 1 2 Ba(IO ) Ba 2 IO Ba IO Ba IO 1.57 10 2 0.200 0.38 0.78 6.79 10 2 0.200 6.79 10 2 0.200 1.698 10 1.698 10 s sp sp K K x x x x x x x x a a g g g g - - - - - - - - - - - + é ù é ù= = ê ú ê úë û ë û é ù é ù= ê ú ê úë û ë û ´ = + ´ = + ´ = + = ´ = ´  7 2+ 7 7 Ba(IO )3 2 Ba 1.698 10 1.7 10M Mc - -é ù = = ´ = ´ê úë û  1c. (10 points) What mass of Ba(IO3)2 can be dissolved in 0.500 L of this 0.200 M KIO3 solution at 25 °C? ( )( )( )7 5 Ba(IO )3 2 1.698 10 0.500 391.136 3.3 10gmolL molm L g - -= ´ = ´ Chemistry 262 Second Hour Exam KEY Spring 2011 Page 2 2a. (20 points) Determine the pH at the equivalence point of a titration of arsenic acid (H3AsO4) with NaOH as indicated in this reaction: Assume that the formal (analytical) concentration of -24HAsO at the equivalence point is 0.070 M. The Ka’s for arsenic acid are: - - -= ´ = ´ = ´3 7 12a1 a2 a35.8 10 , 1.1 10 , 3.2 10K K K . ( )( ) ( ) 4 4 3 HAsO+ 3 HAsO 2 12 14 13 + 3 5 7 + 10 3 H O 1 / 3.2 10 0.070 1.000 10 2.34 10 H O 0.070 6.363 101 1.1 10 H O 6.06 10 9.2175 9.22 a w a K c K c K M pH - - - - - +é ù =ê úë û + ´ + ´ ´é ù = =ê úë û ´+ ´ é ù = ´ê úë û = = 2b. (5 points) If 20.00 mL of the NaOH were required to reach the above indicated equivalence point, what is the pH of the solution after 15.00 mL of NaOH are added? 2010 30 This is the second half-equivalence point: pH = pKa2 = 6.96 2 3 4 4 2H AsO 2 OH HAsO 2 H O - -+ ¾¾ +