1. Assuming that all of the SCN- added to mixture #1 exists as [FeSCN]2+ at equi

Question

1. Assuming that all of the SCN- added to mixture #1 exists as [FeSCN]2+ at equilibrium, determine [FeSCN]2+ eq for mixture #1.
2. Use the absorbable of [FeSCN]2+ eq For mixture #1 to calculate the molar absorption it of [FeSCN]2+
3. Use Beers Law and the molar absorptivity that you calculated from mixture #1 to calculate [FeSCN]2+ eq for mixture #2.
4. Calculate [Fe3+] and [SCN-], for mixture #2.
5. Use your answers to the previous two questions to calculate both [Fe3+] eq and [SCN-]eq for mixture #2.
Volume, ml Fe3+ 10.0 Mix # 1 0.020 M 3.25 x 10-4 0.20 M Fe3+ 10.0 Absorbance SCN- 10.0 10.0 0.708 0.423 2

Explanation / Answer

1. Assuming all of SCN- exists as [FeSCN]2+ at equilibrium for mixture#1,

[FeSCN]2+ eq = 3.25 x 10^-4 M x 10 ml/20 ml = 1.625 x 10^-4 M

2. absorbance = molar absoprtivity x path length x concentration

molar absoptivity of [FeSCN]2+ = 0.708/1.625 x 10^-4 = 4357 M-1.cm-1

3. [FeSCN]2+ for mixture#2 = 0.423/4357 = 9.71 x 10^-5 M

4. For mixture#2

[Fe3+] = 0.02 M x 10 ml/20 ml = 0.01 M

[SCN-] = 3.25 x 10^-4 M x 10 ml/20 ml = 1.625 x 10^-4 M

5. For mixture#2

[Fe3+] eq = 0.01 - 9.71 x 10^-5 = 9.90 x 10^-3 M

[SCN-] = 1.625 x 10^-4 - 9.71 x 10^-5 = 6.54 x 10^-5 M

6. Kf for mixture#2 = [FeSCN]2+/[Fe3+]eq.[SCN-]eq

                              = (9.71 x 10^-5)/(9.90 x 10^-3 x 6.54 x 10^-5)

                              = 150

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