Experimental Determination of an Equilibrium Constant Experimental Detern Report

Question

Experimental Determination of an Equilibrium Constant

Experimental Detern Report Sheet Part A Molar Concentration of Fe(NO3)3 (from reagent bottle) Molar Concentration of KSCN (from reagent bottle) 0.20 M 0.002 M Standard Solution # 4 Vol. of KSCN (ml)0 Moles SCN [SCN] 15 1.0 10*-6 mol 2.0.10A-5 mol 3.0.10A-5 mol 0.104 M 4 M 5 M Absorbance, A 0.141 0.357 0.774 1.176 Attach Plot of A vs. [FeNCS2+], make sure you have instructor's approval before you go on to part B. The graph proved to be linear. Absorbance vs. Concentration of Fe(NCS) y-3947.7x-0.0157 R2-0.9992 Part B. Molar Concentration of Fe(NO3)3 (from reagent bottle) Molar Concentration of KSCN (from reagent bottle) Test Solution # 4 Vol. of Fe(NO3)3 (ml) Fe+l, initial Vol. of KSCN (ml) [SCN], initial Absorbance, A 1 104-5 1-10 5 1-10 5 1-10-5 1.104-5 0.002 0.097 0.004 0.213 0.006 0.341 0.008 0.447 0.010 0.537 Note: The instructions for the lab ask you to just find the initial moles of Fe and SCN for this report, you need to find the molarity, just divide the moles by the final volume of solution (0.020 L). There's also a typo in the instructions, the volume of Fe(NO3)3 in the example should be 0.010 L, not 0.005 L, it's an artifact from where I modified the volumes of reactants but forgot to change the calculation instructions.

Explanation / Answer

1. In the spectrometer, if a dirty cuvet is inserted, the light passing through the cuvet and solution will be fractionally interrupted or reflected away from the receptor, thus causing the machine to interpret that more light has been absorbed by the solution. Thus, absorbance values will come out to be HIGHER than actual.

Higher absorbance reading will imply higer FeNCS2+ concentration, which will give HIGHER values of Kc. However the change will not be very high, as the value of [SCN-] is very high in the solution.

2. Absorbance is an intrinsic property with units cm-1 . The absorbance value has no relation with the dimensions of the cuvet or the path length. It is a constant property for a particular solution.

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