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Katarina Ong
Connor Wilson
Michael Oliver
February 4 & 11, 2014

Investigation 37: What is the Formation Constant
Experiment Background
Fe​3+​(aq)​+ NCS​-​(aq)​ ↔ FeNCS​2+​(aq)
A theater company has recently discovered this reaction and wants to use it to
prepare solutions that look like blood
...
This will help the
company understand the conditions under which to control the reaction
...
This was accomplished by preparing various
solutions of Fe​3+​ and NCS​-​, allowing the solutions to reach equilibrium, then
determining the concentrations of the reactants
...
With this relationship
established, the concentration of FeNCS​2+​ in the solutions of Fe​3+​ and NCS​allowed to equilibrate could be determined
...
With the equilibrium concentrations of all
three reactants determined, it was now possible to calculate the equilibrium
constant, following the formula

1

Materials and Equipment
Materials

Equipment

Prepared FeNCS​2+​ Solutions

3 - 50mL Pipette

2
...
00 × 10​-5​M FeNCS​2+

Lint-free tissue paper

6
...
00 × 10​-5​M FeNCS​2+

6 15mL Test Tubes

1
...
20 × 10​-4​M FeNCS​2+
1
...
00 × 10​-3​M Fe​3+
2
...
00 × 10​-1​M HNO​3
Deionized Water
Method
The first step in determining the formation constant for the reaction
Fe​3+​(aq)​+ NCS​-​(aq)​ ↔ FeNCS​2+​(aq)
was to establish a relationship between concentration and absorbance for
solutions of FeSCN​2+​
...
First, a 3mL cuvette was filled with 1
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This wavelength was determined to be 470nm, as can be seen in
Table 1
...
The absorbance values
noted in Table 2 were collected in this manner
...
It is this value that allows concentration data to
be determined from absorbance data, following the Beer-Lambert law
A = ε bC
Where A is absorbance, ε is the proportionality constant, b is the length of the
light path in the colorimeter, and C is the concentration of the solution being
tested
...

The quantities noted in Table 2 were added to 6 15mL test tubes, one for each
solution, and the reaction was allowed to come to equilibrium for several minutes
...
With this
data, in conjunction with the Beer-Lambert curve, the equilibrium concentration of
FeSCN​2+​ for each solution could be determined, and the concentrations of Fe​3+
and SCN​-​ could also be determined using stoichiometric ratios
...


3

Data Collection
Table 1
...
297

470*


...
051

635


...

[FeSCN

2+

]aq (M)

2+

were transferred into cuvettes, the

Absorbance

blank

0
...
00 × 10−5

0
...
00 × 10−5

0
...
00 × 10−5

0
...
00 × 10−5

0
...
00 × 10−4

0
...
20 × 10−4

0
...
40 × 10−4

0
...


4

Figure 1
...
The slope of the line is the ε value, which will be used in the
Beer-Lambert Law ( A = εbC ) to convert Absorbance to concentration
...
These solutions were placed in a 3mL
cuvette and read in a colorimeter for absorbance
...
00200)(5
...
0)
[Fe

3+

] =
...
00200)(1
...
0)
[KSCN] = 2
...
​This table details the concentrations of Fe​3+​ and SCN​-​ our group prepared, and
the absorbance value collected after the solutions were allowed to reach equilibrium, as
well as the calculated concentrations of FeSCN​2+​
...
00200
M Fe 3+

mL

...
1
M HNO

1

5
...
0

4
...
001

2
...
173

6
...
0

1
...
5


...
00 × 10−4 0
...
95 × 10−5

3

5
...
0

3
...
001

4
...
311

1
...
0

2
...
5


...
00 × 10−4 0
...
41 × 10−4

5

5
...
0

2
...
001

6
...
437

1
...
0

3
...
5


...
00 × 10−4 0
...
97 × 10−4

[Fe

]

3

Data Processing
The absorbance data from the 6 solutions were converted into concentration data using
the ε value found in Figure 1, and using the Beer-Lambert Law
...

ex
...
173 = 2630(1)(C)
C=

0
...
58 × 10−5 M
The Absorbances and Concentration are shown in Table 4
...
​Absorbance values and FeSCN​2+​ concentration for the six prepared solutions
of Fe​3+​ and SCN​-​ allowed to equilibrate
...
173

6
...
209

7
...
311

1
...
371

1
...
437

1
...
517

1
...

ex
...
001
-x

...
0002
-x

...

ex
...
001−x)(
...
58×10−5 )
(
...
58×10−5 ))(
...
58×10−5 ))

K f = 525

7

Table 3
...

Solution

Formation Constant (K)

1

525

2

391

3

476

4

458

5

459

6

486

Figure 2
...
The error bars were
added to show the variation in Kf values (as solution 2 had a lower Kf value than all the
other solutions)
...


8

Conclusion and Evaluation
The problem presented in this Laboratory Investigation was to determine the constant of
formation (K​f​) for the following reaction:
Fe​3+​(aq)​+ NCS​-​(aq)​ ↔ FeNCS​2+​(aq)
The process by which this constant was determined began with the establishment of a
Beer-Lambert curve, allowing concentration and absorbance to be related for solutions
of FeNCS​2+​
...
Armed with the
information regarding the concentration of FeNCS​2+​ in our six prepared solutions, as
well as the initial concentrations of Fe​3+​ and SCN​-​, detailed in Table 3, it was possible to
determine the equilibrium concentrations of FeNCS​2+​, Fe​3+​, and NCS​-​, and from this the
constant of formation for this reaction, following the expression

The final K​f​ value, determined by averaging the K​f​ values across the six trials
performed, was 466
...
It may also be suggested that the final
concentration of FeNCS​2+​ is dependant on the initial concentration SCN​-​ and can be
controlled by the amount of SCN added, since concentration of Fe​3+​ was kept constant
in this case
...
Though the
formation constant should remain constant across various initial concentrations of
reactants, a higher initial concentration of Fe​3+ ​and SCN​-​ will invariably produce a
redder, more blood-like substance
...
The establishment of the Beer-Lambert curve in the first part of the
investigation was based on pre-prepared solutions, so any error introduced at this point
would be unavoidable without producing more accurate dilutions
...
Inaccuracies in buretting the the various solutions into the
test tubes may have led to high or low values for the initial concentrations, compounding
the error as these values were used to calculate the value of K​f​
...

​

10



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