C1=0.002M (SCN concentration) [Fe(SCN)2+] will be the same
V1 – volume of SCN solution (5, 7, 9, 11, 13, 15 ml)
V2=100ml (total volume of solution)
That is how we obtain data ([Fe(SCN)2+]) for calibration curve. It is incorrect to multiple absorbance on volume for finding [Fe(SCN)2+]. Use abovementioned formula.
When we built calibration curve in Excel we used function “Find trend line” on the diagram and click at the option “Show equation on the diagram” The obtained equation is y=0.0003*x-0.000009.
A1=0.002*0/100=0M
A2=0.002*5/100=0.0001M
A3=0.002*7/100=0.00014M
A4=0.002*9/100=0.00018M
A5=0.002*11/100=0.00022M
A6=0.002*13/100=0.00026M
A7=0.002*15/100=0.0003M
Determination of the equilibrium constant
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K=[FeSCN2+]Fe3+*SCN-
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9. The K value decreases from solution B-1 to solution B-5. It is expected that K will be the same because K value does not depend on reagents concentration and depends only on temperature. If the temperature did not change during the experiment, our equilibrium constant should have the same value.
10. When the calibration curve was built we assumed that all SCN- reacts. In fact some content of SCN leaves in the solution unreacted. This can be a source of error. If there were any temperature changes during experiments it could also influence on the K value.
Conclusions
As K>>1 the equilibrium lies to the product in our experiment. Obtained results have to show that K value does not change and remains the same. We did not get such proof because we did not take into account that during drawing calibration curve some mass of SCN leaves in the solution unreacted.
Equilibrium constant (K value) can be defined by measuring products and reactants’ concentration. We used spectrophotometry to find concentrations of the substances in our experiment. For finding connection between absorbance and substance concentration we built calibration curve. Then the equation of calibration curve was found. When we substitute in formula measured absorbance we can calculate unknown Fe(SCN)2+ concentration. Substituting in the formula equilibrium concentrations of product and reactants we can find the K value. So we learned how to use spectrophotometry for measuring substances concentration in the solution and how to calculate the value of equilibrium constant.
