4 Report sheet Colorimetric Determination of an Equilibrium Constantin Aqueous S

Question

4 Report sheet Colorimetric Determination of an Equilibrium Constantin Aqueous Solution Absorbance Equil. [FeNCS M O Equil. [Fe 1.M Equil. ISCN M K (show calculations culate Y Average (show calculations) QUESTIONS 1. How would the accuracy of your determined Keq hange if all of your volume measurements were made e with graduated cylinders rather than pipets? Tre a CC voCM is 2. If all of the not converted completely to FeNCS when the calibration curve was prepared, SCN was would this raise or lower the value of Kea Explain. 3. Use the mean value you determined for K to calculate the SCN concentration in a solution whose ini tial Fe concentration was 4.00x10 2 M and initial scN concentration was 1.00 x 10 3 M. ls all of the scN in the form of FeNCS

Explanation / Answer

1. Calibration

[SCN-] = [FeNCS]2+

Plot,

absorbance vs [FeNCS]2+ (M)

slope = molar absorptivity of [FeNCS]2+

          = 2182 M-1.cm-1

2. Equilibrium constant

[FeNCS]2+eq = Absorbance/molar absorptivity for [FeNCs]2+

[Fe3+]eq = [Fe3+]initial - [FeNCs]2+eq

[SCN-]eq = [SCN-]initial - [FeNCS]2+eq

Keq = [FeNCS]2+eq/[Fe]3+eq.[SCN-]eq

Sample                       1                    2                  3                    4                    5                      6

[Fe3+]initial (M)      1 x 10^-3     1 x 10^-3     1 x 10^-3        1 x 10^-3        1 x 10^-3        1 x 10^-3

[SCN-]initial (M)          0              2 x 10^-4     4 x 10^-4        6 x 10^-4        8 x 10^-4        1 x 10^-3

[FeNCS]2+eq(M)        0             7.3 x 10^-5 1.3 x 10^-4    1.9 x 10^-4     2.6 x 10^-4      3.1 x 10^-4

[Fe3+]eq (M)           1 x 10^-3    9.3 x 10^-4   8.7 x 10^-4   8.1 x 10^-4     7.4 x 10^-4      6.9 x 10^-4

[SCN-]eq (M)               0            1.3 x 10^-4   2.7 x 10^-4   4.1 x 10^-4    5.4 x 10^-4      6.9 x 10^-4

Keq                              0                604                  553            572                 650                651

average Keq              606

1. Accuracy for Keq would be much lower when measuring cyllinder was used instead of pipet as error introduced would be higher with cyllinder

2. If all of SCN- was not converted to [FeNCS]2+ at equilibrium for calibration curve, molar absorptivity calculated would be higher, thus Keq would thus be lower than actual value

3. Keq = 606

            Fe3+      +      SCN-    <===> [FeNCS]2+

I         4 x 10^-2        1 x 10^-3                 -

C             -x                 -x                        x

E      (4x10^-2 - x) (1x10^-3 - x)           x

So,

606 = x/(4 x 10^-2 - x)(1 x 10^-3 - x)

4 x 10^-5 - 0.041x + x^2

606x^2 - 23.85x + 0.024 = 0

x = [FeNCS]2+eq = 1.0 x 10^-3 M

So all of [SCN-] has been converted into [FeNCS]2+ complex.

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