REPORT SHEET EXPERIMENT Colorimetric 22 Determination of an Equilibrium Constant

Question

REPORT SHEET EXPERIMENT Colorimetric 22 Determination of an Equilibrium Constant in Aqueous Solution A. Preparation of the Calibration Curve M Concentration of FeoNo) in o IOM HNO, solution o.2ooMoc 2.box Concentration of NasCN in 0.10 M HNO solution Flask Number Volume of NaSCN, mil. O Solution 00 X Initial [SCN J. M KEquil. LFeNcs? 1. M B. Determination of the Equilibrium Constant Concentration of Fe(NO), in 0.10 M HNO solution 2.O0X1 M concentration of NascN solution 2 DDXLO in 0,10 M HNO Test Tube Number Solution Temperature Volume of FeNO,) solution, mL o I ote 2 3 00 volume of NascN Solution, mL Initial [Fe M 00X10 Initial [scN-1, M Copyright c 2012 Pearson Education, Inc. 287

Explanation / Answer

first you need to plot the callibration curve between the Absorbance and concentration of the standard solution prepared.(Part A)

plz see image

then using the values of absorbance of the unknown samples ,determine the equilibrium concentration of FeSCN2+ from the callibration curve.

you have absorbance along y-axis(the ordinate) ,so find out the corresponding eqm concentration (along x-axis ) or the abscissa

if x=[FeSCN2+]eqm

[Fe3+]eq=[Fe3+]o-x     {[Fe3+]o, [SCN-]o=initial concentrations}

[SCN-]eq=[SCN-]o-x

keq=[FeSCN2+]eqm/([Fe3+]o-x)([SCN-]o-x)=x/([Fe3+]o-x)([SCN-]o-x)

So data is as follows-

<

0.0001641

now ,

for trial 2,

[FeSCN2+eq=x=0.000137 M

[Fe3+]eq=[Fe3+]o-x=0.001M-0.000137 M=0.000863M

[SCN-]eq=[SCN-]o-x=0.0002-0.000137=0.000063M

keq=0.000137M/(0.000863M)(0.000063M)=2519.81=2.519*10^3

keq=2.519*10^3

similarly for trial 3,4...

absorbance [FeSCN2+] 0.475 0.000137 0.552 0.000155 0.586 0.000163 0.59 0.000164 0.591

0.0001641

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