Spectrophotometric determination of an equilibrium constant of FeSCN2+. 1. the s

Question

Spectrophotometric determination of an equilibrium constant of FeSCN2+.

1. the solution used to construct the calibration curve (Beer's Law plot) for FeSCN2+ have an excess of FE3+ and a known initial concentration of SCN-.
Explain why the concentration of
FeSCN2+ can be assumed to equal that of the initial SCN- conc.

2. Calculate the concept of FeSCN2+ I'm standard solution. 4 (use beers law table 1) (show work).

3. calculate the initial (after mixing but before any reaction takes place) concentration of Fe3+ and SCN- for solution 3 of procedure part 2. (use Kc table 2 from page 2) (show work)

4. if the equilibrium concentration of FeSCN2+ is found to be 5.00x10^-5M using solution 3 of procedure part 2 calculate the equilibrium conc of Fe3+,SCN- and the resulting equilibrium constant for the reaction. (show equilibrium set up and your work)

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

...hopefully the charts are legible.

TAElF5 NIFDED for pre lab TABLE 1: Solutions for Beer's Law calibration Plat 1 lalank 25.0 25.0 73.0 25.0 71.0 25.0

Explanation / Answer

for the given reaction,

Fe3+ + SCN- --> FeSCN-

For calibration curve

Table

Std.            [Fe3+] (M)                   [SCN-] (M)

1         0.2x25/100=0.05                     0

2         0.2x25/100=0.05         0.002x1/100=2x10^-5

3         0.2x25/100=0.05         0.002x2/100=4x10^-5

4         0.2x25/100=0.05         0.002x4/100=8x10^-5

5         0.2x25/100=0.05         0.002x6/100=1.2x10^-4

6         0.2x25/100=0.05         0.002x8/100=1.6x10^-4

Here,

SCN- is the limiting reagent so, when all of SCN- is consumed in the reaction for the formation of [FeSCN]2+ complex, the reaction stops.

Plot, [FeSCN]2+ (or SCN-) on x-axis vs absorbance for each solution on y-axis

The slope of the curve = molar absorptivity for [FeSCN]2+

For Keq calculation

Table

Std.          initial [Fe3+] (M)              initial [SCN-] (M)

blank         0.002x5/10=0.001                         0

  1             0.002x5/10=0.001         0.002x1/10=2x10^-4

  2             0.002x5/10=0.001         0.002x2/10=4x10^-4

  3             0.002x5/10=0.001         0.002x3/10=6x10^-4

  4             0.002x5/10=0.001         0.002x4/10=8x10^-4

  5             0.002x5/10=0.001         0.002x5/10=1x10^-3

For these samples, find absorbance for the 6 solutions

Using molar absorptivity found by caibration curve, determine,

[FeSCN]2+eq = absorbance (for each solution)/molar absorptivity

Now Calculate,

[Fe3+]eq = initial [Fe3+] - [FeSCN]2+eq

and,

[SCN-]eq = initial [SCN-] - [FeSCN]2+eq

From these three equilibrium values calculate Keq,

Keq = [FeSCN]2+eq/[Fe3+]eq.[SCN-]eq

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