S. SMITH
NAME:
SAMPLE
DATE:
October, 2011
TITLE:
The Equilibrium Constant
PURPOSE:
In this experiment I calculated the equilibrium constant for the reaction
shown below under different conditions to determine if the equilibrium
constant is really constant.
Fe3+ (aq) + SCN– (aq)  FeSCN2+ (aq)
PROCEDURE:
This lab, The Equilibrium Constant, was provided by the teacher, who got it
from Flinn ChemTopic Labs Volume 15: Equilibrium, Irene Cesa, Senior
Editor, Flinn Scientific, Inc., Batavia, IL, © 2003, pp. 29-44.
In Part A of the lab, I prepared six solutions – including a reference solution
(known concentration) and a series of five test solutions (varied in
concentration of SCN– (aq)). In Part B of the lab, I measured the absorbance
of the solutions using a Spec 20. After collecting the data, I calculated the
unknown equilibrium concentrations of the FeSCN2+ (aq) which I then used
to determine the Keq.
DATA COLLECTION combined with
DATA PROCESSING & PRESENTATION:
Table 1: Preparing the Solutions
Reagents (Solutions)
0.0020 M
0.0020 M SCN– Distilled H2O
+/-– .0005 M*
+/-– .0005 M*
(25% error )
(25% error )
* uncertainty due to solution prep
Volume of Solution Used
+/-– 0.005 mL** +/-– 0.005 mL** +/-– 0.005 mL**
(0.1% error)
(0.1% error)
(0.1% error)
5.00
1.00
4.00
5.00
2.00
3.00
5.00
3.00
2.00
5.00
4.00
1.00
5.00
5.00
0.00
**uncertainty of graduated pipette / bulb system
Fe3+
Test Solution # 1
Test Solution # 2
Test Solution # 3
Test Solution # 4
Test Solution # 5
NOTE: It is clear that the uncertainty of the solution prep is very high… and will far exceed
any uncertainty due to measuring with the pipette.
Table 2: Initial Concentration and Measured Absorbance of Solutions
Absorbance
Reagents
Temperature of Solutions
(assumed to be room temperature):
Solution 
Uncertainty 
Test Solution # 1
Test Solution # 2
Test Solution # 3
Test Solution # 4
Test Solution # 5
Reference
Solution # 6
[ SCN– ]
in M
+/- 25 %
2.0 x 10–4
4.0 x 10–4
6.0 x 10–4
8.0 x 10–4
1.0 x 10–3
[ Fe3+ ]
in M
+/- 25 %
1.0 x 10–3
1.0 x 10–3
1.0 x 10–3
1.0 x 10–3
1.0 x 10–3
0.180
2.0 x 10–4
+/- 25 %
+/- 0.01
+/- 2.5 % (avg)
0.18 +/- 6 %
0.32 +/- 3 %
0.46 +/- 2 %
0.60 +/- 2 %
1.15 +/- 0.9 %
1.55 +/- 0.6 %
Sample Calculations for Test Solution # 1
[ Fe3+ ] = (0.0020 M) (5.00 mL) / (10.00 mL) = 1.0 x 10–3
Uncertainty = 25% + 0.1% + 0.1% = 25.2 % (essentially 25% !)
[ SCN1– ] = (0.0020 M) (1.00 mL) / (10.00 mL) = 2.0 x 10–4
Uncertainty = 25% + 0.1% + 0.1% = 25.2 % (essentially 25% !)
Sample Calculations for Reference Solution # 6
[ Fe3+ ] = (0.200 M) (9.00 mL) / (10.00 mL) = 0.18M
Uncertainty = 2.5% + 0.1% + 0.1% = 2.7 %
[ SCN1– ] = (0.0020 M) (1.00 mL) / (10.00 mL) = 2.0 x 10–4
Uncertainty = 25% + 0.1% + 0.1% = 25.2 % (essentially 25% !)
Table 3: Calculations of the Equilibrium values of Ions in Solutions
Solution 
Ions in Solution at Equilibrium
[ FeSCN2+ ]eq
[ Fe3+ ] eq
[ SCN– ] eq
in M
in M
in M
Keq
Uncertainty 
Test Solution # 1
Test Solution # 2
Test Solution # 3
Test Solution # 4
Test Solution # 5
Reference Solution
2.3 x 10–5 M
+/- 32%
4.1 x 10–5 M
+/- 29%
5.9 x 10–5 M
+/- 28%
7.7 x 10–5 M
+/- 28%
1.5 x 10–4 M
+/- 27%
2.0 x 10–4 M
+/- 25 %
9.77 x 10–4 M
+/- 25%
9.59 x 10–4 M
+/- 25%
9.41 x 10–4 M
+/- 25%
9.23 x 10–4 M
+/- 25%
8.5 x 10–4 M
+/- 25%
1.77 x 10–4 M
+/- 32%
3.59 x 10–4 M
+/- 17%
5.41 x 10–4 M
+/- 12%
7.23 x 10–4 M
+/- 10%
8.50 x 10–4 M
+/- 10%
2.0 x 10–4 M
+/- 11%
Average Value
133
+/- 89%
119
+/- 71%
116
+/- 65%
115
+/- 63%
208
+/- 62%
138
+/- 70%
Sample Calculations
Reference Solution [ FeSCN2+ ]eq = 2.0 x 10–4 M +/- 25 %
Because we infer that an excess of the [ Fe3+ ] will shift the
equilibrium in such a way that all of the [ SCN1– ] is converted.
Test Solution 1 [ FeSCN2+ ]eq
= (A1/Aref) x [ FeSCN2+ ]ref
= (0.18 / 1.55) x 2.0 x 10–4 M = 2.3 x 10–5 M
Uncertainty = 6% + 0.6% + 25% = 31.6% = 32%
= [ Fe3+ ]initial – [ FeSCN2+ ]eq
= 1.0 x 10–3 – 2.3 x 10–5 = 9.77 x 10–4 M
Uncertainty = 0.00025 M + 0.0000074 M ~ 0.00025 M = 25%
Test Solution 1 [ Fe3+ ]eq
Test Solution 1 [ SCN1- ]eq = [SCN1-]initial – [ FeSCN2+ ]eq
= 2.0 x 10–4 – 2.3 x 10–5 = 1.77 x 10–4 M
Uncertainty = 0.00005 M + 0.0000074 M = 0.0000574 M ~ 32%
Test Solution 1 Keq =
=
=
Uncertainty =
____[ FeSCN2+ ]eq____
[ Fe3+ ]eq x [ SCN1- ]eq
( 2.3 x 10–5 M ) / (9.77 x 10–4 M ) (1.77 x 10–4 M)
133
32% + 25% + 32 % = 89%
Percent Error Calculation
[ (138 – 150) /150 ] x 100 = 8.0%
CONCLUSION:
In this lab, the equilibrium constant for the following reaction:
Fe3+ (aq) + SCN– (aq)  FeSCN2+
(aq)
was determined to be 138 +/- 70% or 138 +/- 97. This means that the Keq
value could have been as low as 41 or as high as 235. Although my results
for each trial varied from 115 to 208, each trial’s Keq is well within the
uncertainty. The standard deviation XXXX This means that the equilibrium
constant is, indeed, constant… within the parameters of this experiment.
Unfortunately the 70% uncertainty makes it difficult to really value this
information. (see evaluation for more discussion)
The actual value expected for this reaction is 150, as provided by my
instructor who obtained this information from the teacher’s guide of the lab
book. My value of 138 is within 8.0 % of the ‘known’ value, indicating a
moderate degree of accuracy – well within the 70% uncertainty.
Note: Units for the Keq would be M. I choose not to report these units as
they seem nonsensical.
EVALUATION:
SOURCES OF ERROR and SUGGESTIONS FOR IMPROVEMENT:
The primary source of error in this lab seems to be due to random
error. The majority of the 70% error stems from the 25% uncertainty in the
stock solution of the 0.0020 M Fe3+ and 0.0020 M SCN–. Part of this
uncertainty was ‘estimated’ by the teacher as a combination of 2 factors:
the solutions were stock solutions on the shelf – probably sitting for more
than 9 months AND the gram amounts of initial compound were measured
using a mg balance (0.001g) instead of an analytical balance (0.0001g). If
these solutions had been prepared more recently and more carefully
(perhaps using an analytical balance) the uncertainty could have been
significantly reduced. Reducing the uncertainty by a factor of 10 could have
put the uncertainty near the range of the experimental error (7% random
error compared to 8% experimental error).*
*IF this had happened I would be looking at an experimental error just
slightly higher than the random error, which would mean I would need to
explain the somewhat lower results. The next most likely reason for error is
in the mixing of the solutions. Although graduated pipettes are more
accurate than graduated cylinders, this was the first time that I had used
this equipment and am not convinced I was totally accurate and precise in
my handling of the bulb and pipette combination. If I delivered less of the
solution than I intended, it would cause less of the colored FeSCN2+ (aq) to
be produced… resulting in a lower Keq overall. If I delivered more of the
solution than I intended, it would cause more of the colored FeSCN2+ (aq) to
be produced… resulting in a higher Keq overall. I do not think there were
any systematic errors, as the directions were easy to follow and the other
equipment was easy to use with minimal practice. The temperature was
recorded and did not change therefore had no impact.