This question is about Job\'s Method. And determining the empirical formula of a
ID: 1065569 • Letter: T
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This question is about Job's Method. And determining the empirical formula of an iron complex. Create the formula to find the MOL FRACTION OF Fe.
The variables I can use are Fe (mL), SCN (mL), and Absorbance. (No data for these variables/data sets, since the lab hasn't been done).
The procedure used for your experiment based on the limiting reactant principle and called Job's Method of Continuous Variation. A series of solutions is prepared with varying concentrations of ligand and metal ion ratio. Each solution is analyzed to determine the ratio that corresponds to the highest coordination complex concentration. A plot of absorbance versus mol fraction of nd will be used to determine the ligand concentration corresponding to the maximum concentration of complex ion. The concentration of the complex e examining is depicted by the following reaction: [Fe (SCN) n SCN is your metal cation, and thiocyanate, SCN is your ligand. You will where iron (III), Fe first determine the mol fraction of ligand present in each of your solutions. mol SCN mol fraction of ligand mol SCN- mol Fe With the mol fraction of the ligand and the measured absorbance of each solution a graph of Absorbance versus Mole Fraction of Ligand can be plotted to determine the ScN corresponding to the maximum concentration of [Fe(scN)] 3 n 250 As demonstrated by Figure 11.1, MicroLab TM can be used to extrapolate two linear portions of the curve until they intersect. The two resulting equations can be used to determine the intersection point. Thusly, the mol fraction of SCN and the mol fraction of Fe s determined at the maximum absorbance Given that the graph in Figure 11.1 signifies the experimental empirical formula of the iron complex formed, +3-n solving the two equations for x will yield the value [Fe(SCN)n] of the mol fraction of SCN in the complex. y 2.5963x 2.7203 y 2.6188x 0.1608 2.6188x 2.5963x 2.7203 0.1608 0.49079 The mol fraction of Fe s calculated as follows: 0.49079 0.50921 The empirical formula of the iron complex, [Fe(scN)n] +3-n s determined by the ratio of mol fraction of SCN to the mol fraction of Fe The value of n is calculated as follows:Explanation / Answer
You already have the set of formula for calculating n. I can tell you how to go about the experiment.
You are given a stock solution of Fe3+ of say M1 mol/L concentration and a ligand stock of M2 mol/L. Suppose you mix the reactants in different proportions as per the table below and obtain the mole fraction of the ligand as below:
Vol. of M1 mol/L Fe3+ (mL)
Vol. of M2 mol/L SCN- (mL)
Moles of Fe3+ = (vol. in L)*(concentration in mol/L)
Moles of SCN- = (vol. in L)*(concentration in mol/L)
Mole fraction of SCN- (call this XL)
Absorbance
x1
y1
(x1M1/1000)
(y1M2/1000)
(y1M2/1000)/[(x1M1/1000) + (y1M1/1000)] = (y1M2)/(x1M1 + y1M2)
A1
x2
y2
(x2M1/1000)
(y2M2/1000)
(y2M2)/(x2M1 + y2M2)
A2
x3
y3
(x3M1/1000)
(y3M2/1000)
(y3M2)/(x3M1 + y3M2)
A3
x4
y4
(x4M1/1000)
(y4M2/1000)
(y4M2)/(x4M1 + y4M2)
A4
The above table gives the mole fraction of the ligand and the corresponding absorbances. A plot of absorbance vs mole fraction of the ligand is drawn. The plot contains two lines which meet at the maximum absorbance of the complex vs the mole fraction of the ligand (as you can see from the figure given in your text).
The rest portion is pretty easy. You set the equations for the two lines at the maximum absorbance to be equal to each other and calculate the mole fraction of the ligand.
You find the value of XL to be a finite fractional number.
Since (mole fraction of Fe3+) + (mole fraction of SCN-) = 1 and you know the mole fraction of the ligand (by solving the equations), you can calculate the mole fraction of Fe3+. That is
XM + XL = 1 (XL = XSCN- = 0.49079 as per the given theory). Plug the value of XSCN- in the above equation to find XM ( = XFe3+).
Then take the ratio n = (mole fraction SCN-)/(mole fraction of Fe3+),i.e.,
n = XSCN-/XFe3+.
The value should be close to 1.
Vol. of M1 mol/L Fe3+ (mL)
Vol. of M2 mol/L SCN- (mL)
Moles of Fe3+ = (vol. in L)*(concentration in mol/L)
Moles of SCN- = (vol. in L)*(concentration in mol/L)
Mole fraction of SCN- (call this XL)
Absorbance
x1
y1
(x1M1/1000)
(y1M2/1000)
(y1M2/1000)/[(x1M1/1000) + (y1M1/1000)] = (y1M2)/(x1M1 + y1M2)
A1
x2
y2
(x2M1/1000)
(y2M2/1000)
(y2M2)/(x2M1 + y2M2)
A2
x3
y3
(x3M1/1000)
(y3M2/1000)
(y3M2)/(x3M1 + y3M2)
A3
x4
y4
(x4M1/1000)
(y4M2/1000)
(y4M2)/(x4M1 + y4M2)
A4
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