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

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⁺ + Cr₂O₇^2- + 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

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