To calculate the equilibrium concentration, you can use the ICE table approach.

Question

To calculate the equilibrium concentration, you can use the ICE table approach. ICE stands for "initial" (I), "change" (C) and "equilibrium" (E). The use of ICE tables may have been covered in your lecture. If not, please review the background knowledge posted on iCollege. Answer the following questions. In an experiment of determining the equilibrium constant of ferric thiocyanate, a student group got the absorbance vs [FeSCN^2+] as shown below In a test equilibrium system, the student group got an absorbance, 0.32. Initially, they used [Fe^3+] = 0.0010M, [SCN^-] = 0.00060M. Calculate the equilibrium constant of the test system. You can use the following steps to solve the problem. a. Use the absorbance vs [FeSCN2^+] linear equation to determine the concentration of [FeSCN2^+], when its absorbance is 0.32. b. Use the ICE table to determine the equilibrium concentration of [Fe^3+] and [SCN^-].

Explanation / Answer

Given that absorbance = 0.32

from the fit equation of the calibration curve, we can calculate the concentration of the Iron SCN complex.

y = 4312 x + 0.0075

0.32 = 4312 x + 0.0075

x = 7.5 x 10-5 M

b) Initial concentrations are given

Fe3+ + SCN- in equilibrium with [FeSCN]2+

Thus one mole Fe2+ reacts with one mole SCN-

change in concentration is 7.5 x 10-5 M

Remaining amount of Fe2+ = 0.001-7.5 x 10-5 = 9.27 x 10-4

Remaining amount of SCN- is = 0.0006-7.5 x 10-5 = 0.000525

Now we found the equilibrium concentrations

K = [FeSCN]2+/[Fe3+] [SCN-] = (7.5 x  10-5)/ (0.000525) 9.27 x 10-4 = 154.11

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