Fingerprint smudges are present on the covets containing the solution placed int

Question

Fingerprint smudges are present on the covets containing the solution placed into the spectrophotometer for analysis. How does this technique error affect the absorbance reading for FeNCS^2+ in the analysis? Explain. Will the equilibrium concentration of FeNCS^2+ be recorded as being too high or too low? Explain. Will the equilibrium concentration of SCN^- be too high, too low, or unaffected by the technique error? Explain. Will the k_c for the equilibrium be too high, too low. or unaffected by the technique error ? Explain.

Explanation / Answer

Fingerprints on a cuvette would affect absorbance and concentration calculation by the fact that fingerprints absorb and scatter light slightly, despite not be readily visible. A cuvette with fingerprints on it will give a slightly higher absorbance reading and the measured concentration will be inaccurate.

b) The the reaction for above example

Fe3+(aq) + SCN-(aq) -----> FeSCN2+(aq)

As we are measuring the absorbance of FeSCN+2 in the solutions at equilibrium. But if we measured the absorbance it is erroneously high due to the fingerprints, then our calculated equilibrium concentration of FeSCN+2 will end up being significantly higher than the actual concentration. Now, the amounts of Fe+3 and FeSCN+2 at equilibrium must add up to the original amount of Fe+3 added to the solution. This means that your calculated equilibrium concentration of Fe+3 will end up being erroneously low.

c) Similarly, the amount of free and bound SCN- must add up to the original amount of SCN- added to the solution, so your calculated equilibrium concentration of SCN- will also be erroneously low.

d) From the above reaction, for which the equilibrium constant expression is Kc = [FeSCN+2]/([Fe+2][SCN-]). If we plug the bad data into this expression, the denominator will be lower and the numerator will be higher, so the end result is that we will get an erroneously high value for Kc.

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