In an experiment of determining the equilibrium constant of ferric thiocyanate,
ID: 475144 • Letter: I
Question
In an experiment of determining the equilibrium constant of ferric thiocyanate, a student group got the absorbance vs [FeSCN^2+] as shown below. Absorbance vs[FeSCN^2+] In a test equilibrium system, the student group got an absorbance, 0.32. Initially, they used [Fe^3+] = 0.0010 M, [SCN^-N] = 0.00060 M. Calculate the equilibrium constant of the test system. You can use the following steps to solve the problem. Use the absorbance vs [FeSCN^2+] linear equation to determine the concentration of [FeSCN^2+], when its absorbance is 0.32. Use the ICE table to determine the equilibrium concentration of [Fe^3+] and [SCN^-].Explanation / Answer
From the graph,
slope = molar absorptivity for [FeSCN]2+
= 4312
a. concentration [FeSCN]2+ = 0.32/4312 = 7.42 x 10^-5 M
b. ICE chart
[Fe3+] [SCN-] [FeSCN]2+
I 0.001 0.0006 -
C 7.45 x 10^-5 7.45 x 10^-5 7.45 x 10^-5
E 0.001-7.45 x 10^-5 = 9.25 x 10^-4 0.0006-7.45 x 10^-5 = 5.25 x 10^-4 7.45 x 10^-5
Equilibrium constant K = [FeSCN]2+[Fe3+][SCN-]
= (7.45 x 10^-5)/(9.25 x 10^-4)(5.25 x 10^-4)
= 153.41
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