Question

1) Frank and Oswalt report a molar absorptivity of 4700 L mol^-1 cm^-1 for thiocyanatoiron(lll) ion. What absorbance would you expect for a soloution that it 1.0e-4 M in thiocyanatorion(lll) ion, if the path length is 1.00 cm?

2) At a given temperature, the equilibrium constant for the system studied in this experiment is 1.40e+2. Suppose 100.0 ml of 2.00e-3 M KSCN was mixed with 100.0 ml of 2.00e-3 M Fe(NO_3_)_3_. At equilibrium, what molar concentration of the thiocyanatoiron(lll) ion would you expect?

Answer 1

A = abc
A = 4700*1*1E-4
Solve for absorbance.

100 mL x 2E-3 = 0.2 millioles Fe^3+
100 mL x 2E-3 = 0.2 millimols SCN^-
(Fe^3+) = 0.2 mmols/200 mL = 0.001M
(SCN^-) = likewise = 0.001M
.......SCN^- + Fe^3+ ==> FeSCN^2+
.....0.001....0.001.......0
C......-x.......-x.........x
E....0.001-x..0.001-x......x

Keq = 140 = (FeSCN^2+)/(SCN^-)(Fe^3+)
Substitute from the E line and solve for x

Answer 2

2.00x10^-3