REPORT SHEIT EXPERIMENT Colorimetric 22 Determination of an Equilibrium Constant
ID: 1033147 • Letter: R
Question
REPORT SHEIT EXPERIMENT Colorimetric 22 Determination of an Equilibrium Constant in Aqueous Solution A. Preparation of the Calibration Curve Concentration of FetNO,h, in o. IO M HNO, solutionM Concentration of NaSCN in 010 M HINO; solution M Flask Number Volume of NaSCN, ml Initial [SCN L.A Equil. [FeNCS.M Percent T Absorbance B. Determination of the Equilibrium Constant Concentration of FetNO,), in 0,10 MIINO, solution0.3m Concentration of NasCN in 0.10 M HNO, solution ? ??5 n Test Tube Number 22.22 Solution Temperature volume of Fe(NO,) Solution, ml. Volume of NaScN nl imL LmL Solution, mL Initial [F-LM OKI 081-0001 0.001 93 Copyright © 2015 Pearson Education, Inc.Explanation / Answer
A. calibration
From the given data
plot absorbance vs [FeSCN]2+
the slope = 5430 M-1.cm-1
B. Keq calculation
Test tube 1,
[FeSCN2+]eq = 0
Keq = 0
Test tube 2,
[FeSCN2+]eq = 0.385/5430 = 7.1 x 10^-5 M
[Fe3+]eq = 0.001 - 7.1 x 10^-5 = 9.3 x 10^-4 M
[SCN-]eq = 2 x 10^-4 - 7.1 x 10^-5 = 1.3 x 10^-4 M
Keq = (7.1 x 10^-5)/(9.3 x 10^-4)(1.3 x 10^-4) = 592
Similarly for,
Test tube 3,
[FeSCN2+]eq = 1.3 x 10^-4 M
[Fe3+]eq = 8.7 x 10^-4 M
[SCN-]eq = 2.7 x 10^-4 M
Keq = 553
Test tube 4,
[FeSCN2+]eq = 2 x 10^-4 M
[Fe3+]eq = 8.0 x 10^-4 M
[SCN-]eq = 4.0 x 10^-4 M
Keq = 625
Test tube 5,
[FeSCN2+]eq = 2.6 x 10^-4 M
[Fe3+]eq = 7.4 x 10^-4 M
[SCN-]eq = 5.4 x 10^-4 M
Keq = 651
Test tube 6,
[FeSCN2+]eq = 3.0 x 10^-4 M
[Fe3+]eq = 7.0 x 10^-4 M
[SCN-]eq = 7.0 x 10^-4 M
Keq = 612
average Keq = 505.5
Questions
1. Accuracy would reduce as graduated cyllinders are less accurate than pipet.
2. if all of SCN- is not converted to complex, the resulting Keq would be lower than expected value.
3. Keq = [FeSCN2+]/[Fe3+][SCN-]
let x amount of complex is formed
then,
505.5 = x/(0.04 - x)(0.001 - x)
505.5x^2 - 22x + 0.02022 = 0
x = 9.4 x 10^-4 M
So equilibrium [SCN-] = 9.4 x 10^-4 M
4. not all of SCN- is converted into complex. Initial SCN- is much greater than equilibrium SCN-.
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