I just completed a lab where I evaluated the equilibrium constant for the reacti

Question

I just completed a lab where I evaluated the equilibrium constant for the reaction of iron(III) with thiocyanate ion. The reaction was Fe^3+ + SCN- --> FeNCS^2+ . I gathered a couple of solutions with varying concentrations of SCN and a constant concentration of Fe^3+. I used a spectrophotometer, an insturment designed to display the amount of light abosrbed by the solution, which ultimately gave me the percent transmittance (%T) reading (which indicates the amount of light passing through the solution and reaching the detector). I then calculated the equivalent absorbance for each solution using A= 2.00-log(%T). Using absorbance, and the slope I calcualted from a given graph, I was able to determine the concentration of FeNCS^2+.

a). would the measured absorbance of an FeNCS2+ ion solution be higher, lower, or unaffected if there were fingerprints on the cuvette? Briefly explain

b). how would the error described above affect calculated FeNCS2+ ion concentration in the solution? Explain

c). how would the error described above affect the calculated constant equilibrium? Explain

I calculated the constant equilibrium to be about 200. Which is high, meaning the reaction is product favored.

thanks for the help and please be thorough!!

Explanation / Answer

a) Absorbance wil be higher than expected , some part of light would be absorbed by finger prints

Out of remaining part some part would be absorbed by complex. Since absorbances are additive . Total absorbtion increases

b) Absorbance = e x Conc where e= molar absoerbitivity const,

as Absrobance increases conc of complex FeNCS2+ increases

c) calculated K eq will be higher than original value

since Ke = [FeNCS2+]/[Fe3+][SCN-]  

as complex conc increase K eq will also increase

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