Question

These post lab questions are regarding the spectroscopic determination of an equilibrium constant of an FeSCN complex where the wavelength was set at 447 nm. These are general questions about the effects on calculations each scenario would have... Sooooo

1. What would have happened to your absorbance reading and to your calculations of Keq if the spectrophotometer had been set at 520 nm rather then 447 nm? ) Clearly explain

2. How would your calculations of the concentration of FeSCN been affected if the cuvette had a 1.5 path length rather than a 1.0 cm path length you were told to use.

3. Finally, Would your equilbirium constant results been affected if you had swapped the volumes used of Fe and SCN?

Answer 1

1.in our experiment(lamds max) λ_max=447 nm

λ_max(lambda max is the wavelength at which the absorbance is maximum.The extent to which a sample absorbs light depends strongly upon the wavelength of light also ,the absorbance spectrum shows how the absorbance of light depends upon the wavelength of the light.

therefore change in lambda max will lead to deviations from Beers law

Though both 520nm and 447nm fall in the visible range of spectrum there will be uncertainity in the absorbance.

2.The beers law stated that A=ebc

where e is the molar absorbtivity with units of L mol^-1 cm^-1

b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained

c is the concentration of the compound in solution, expressed in mol L^-1

A = ebc tells us that absorbance depends on the total quantity of the absorbing compound in the light path through the cuvette

therfore Absorbance is dirctly proportional to pathelength of the cuvette.