CHEMISTRY 31

Fall, 2016 - Dixon

Homework Set 2 (for Exam 2)

Chapter                                       Problems                                              Date to finish

Set 2.1                                                                                                          Oct. 19

Ch. 6                                             19, 23, 25, 33, 36, 44, 49

Ch. 7                                             2, 7, 15, 18, 26

Set 2.1 text solutions

Additional Problem 2.1.  A 100.0 mL sample of spring water was treated to convert any iron present to Fe2+.  Addition of 25.00 mL of 0.02517 M K2Cr2O7 resulted in the reaction:

6Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O

The excess K2Cr2O7 was back-titrated with 8.53 mL of 0.00949 M Fe2+ solution.  Calculate the concentration of iron in the sample in parts per million.  (5 pts)

Set 2.2                                                                                                          Nov. 2

Ch. 18                                           1, 3, 4, 11, 18

Ch. 23                                           3, 8a, 18, 20, 23, 28, 32, 44

Set 2.2 text solutions

Additional Problem 2.2.  The concentration of iron(III) in a solution is determined by formation of an Fe(SCN)2+ complex.  75.0 mL of a sample containing Fe3+ is reacted with 25.0 mL of 0.100 M NaSCN forming 100.0 mL of solution.  An aliquot (from the 100.0 mL of solution) is placed in a 0.500 cm pathlength cell and the absorbance is measured at 580 nm to be 0.807.

The following important information for this problem was omitted: The K (equilibrium constant) for formation of FeSCN2+ from Fe3+ and SCN- is 1047.

a) Given that the absorptivity of Fe(SCN)2+ at 580 nm is 6,971 M-1 cm-1, calculate the concentration of Fe(SCN)2+ in M in the aliquot.

b) Calculate the concentration of uncomplexed Fe in the aliquot (you can assume a negligible amount of SCN- reacts to produce the Fe(SCN)2+ ion).

c) Calculate the concentration of Fe3+ in M in the original 75.0 mL of solution. (5 pts)

Set 2.3                                                                                                          Nov. 7

Ch. 8                                             1, 4, 5a,b, 10, 13