A student prepared the following 2 mixtures and recorded their absorbances in a

ID: 948858 • Letter: A

Question

A student prepared the following 2 mixtures and recorded their absorbances in a cuvette with a 1.00 cm path length:

1. Write the balanced equilibrium reaction of Fe3+ (aq) with SCN- (aq) to form [FeSCN]2+ (aq).

2. Write the equilibrium formation constant equation for the reaction (Kf expression).

3. Assuming that all of the SCN - added to the mixture #1 exists as [FeSCN]2+ at equilibrium, determine [FeSCN2+]eq for mixture #1.

Volume, mL 0.020 M 3.25 x 104 M Absorbance SCN- 10.0 10.0 0.O20MTa254MI Absorbance Mix # | 0.20 M Fe3+ 10.0 Fe3+ - 0.708 0.423 10.0

1. Balanced equation : Fe3+(aq) + SCN-(aq) ---> FeSCN^2+(aq)

2. Equilibrium constant expression,

Keq = [FeSCN^2+]/[Fe3+][SCN-]

3. With all SCN- added goes to FeSCN^2+ we get,

[FeSCN^2+]eq = [SCN-] = 3.25 x 10^-4 M x 0.01 ml/0.02 ml = 1.625 x 10^-4 M

4. absorbance A = ebc

with,

b = 1 cm

For mixture#1

0.708 = e x 1 x 1.625 x 10^-4

molar absorptivity e = 4357 M-1.cm-1 for [FeSCN]^2+

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