The purpose should include a majority of the topics and techniques covered in the experiment. The procedure section should cite the lab manual and include any changes made to the procedure during lab work.

The data section for this experiment will include two sections one for the calibration data (determination of the extinction coefficient) and one for the equilibrium data (determination of the equilibrium constant). For the calibration data include a table with the solution number A-E, the volume of  Fe³⁺, the volume of KSCN, the total volume, [Fe³⁺], [KSCN] and the absorbance values.

Use the following method to determine [Fe³⁺]:

Solve for the number of moles of Fe³⁺ in each run:

Then divide that number of moles by the total volume of each solution in each run (this should be 10 mL or 0.01L):

Repeat these calculations to determine the [KSCN].

For the equilibrium data solutions F-J, consider the following chemical equation and reaction:

Where x is 1,2 or 3. In order to determine the value of x, we will need to complete the following table for each x value and then determine the set of data with the most constant K value. The data with the most constant K value is the one with the correct value for x. Complete the following table for x = 1, 2 and 3:

Soln	Initial Volume of Fe3+ (mL)	Initial Volume of KSCN (mL)	Total Volume  (mL)	Initial [Fe3+] (M)	Initial [SCN-] (M)	abs	[Fe(SCN)x(3-x)] (M)	Eq [Fe3+] (M)	Eq [SCN-] (M)	K
F
G
H
I
J

The initial concentrations are found the same way as above. The absorbance value was recorded in lab. The [Fe(SCN)_x^(3-x)] can be found using Beer’s Law:

where A is the absorbance value, e is the molar absorptivity or extinction coefficient which will be found from the slope of your calibration graph in Part I, b is the path length of the spectrometer and is given as 1 cm and c will be the concentration of  [Fe(SCN)_x^(3-x)].  To determine the Equilibrium concentrations of [Fe³⁺] and [SCN^-] you will need to use the following two equations where x = 1,2 or 3:

and

where you simply subtract the initial concentration from the equilibrium concentration of [Fe(SCN)_x^(3-x)]

After determining the concentrations at equilibrium determine K for each x value and reaction solution using the equation:

Finally for each x value or set of data calculate the average K and the standard deviation in K. The x value with the smallest variation in K is the value you want. Be sure to state that value and plug it into the reaction equation.

 

The calculation section should include a calibration graph of the concentration vs. absorbance from part 1. All graphing guidelines apply. The slope of this graph is the molar absorptivity or extinction coefficient. Also be sure to include sample calculations for the concentrations used in the calibration graph and each calculation done for your equilibrium data.

 

In the conclusion section include several paragraphs discussing the value for x that you determined and how you reached your conclusion. Also be sure to include the average values of K in your discussion. Be sure to include information about your calibration curve and the value of the molar absorptivity or extinction coefficient. Compare and contrast the values calculated when each of the three values for x are used. Do not forget to discuss any errors.

Answer the following questions:

1. How constant were your Kc values at room temperature? Explain any variation.


2. See if you can find literature and/or Internet references for the equilibrium constant for this equilibrium. Please cite the reference(s). How does your result compare with the one(s) you cited? Perhaps you will even find a similar experiment, with sample results, posted out on the Web.