Determination of an Equilibrium Constant
𝐹𝑒 3+ (𝑎𝑞) + 𝐻𝑆𝐶𝑁 (𝑎𝑞) ↔ 𝐹𝑒𝑆𝐶𝑁 2+ (𝑎𝑞) + 𝐻 + (𝑎𝑞)
𝑘𝑒𝑞 =
[𝐹𝑒𝑆𝐶𝑁 2+ ][𝐻 + ]
[𝐹𝑒 3+ ][𝐻𝑆𝐶𝑁]
Victoria Alexis Treto
02/22/2018
Che122, General Chemistry 2
Spring 2018
Prof. L. Wojciechowicz
Introduction
The purpose of this lab is to evaluate the equilibrium constant of the reaction
between ferric ions and hydrogen thiocyanate by determining the concentration of
Procedure
Preparation of the HSCN Solution
Step1:
 From a repipet bottle, 10.0ml of a 0.01M KSCN solution was added to a
clean 50.00ml volumetric flask.
 This was then diluted to the mark with 0.5M HNO3, mixed well, and
transferred to a labeled beaker.
Preparation of the Beer’s Law Calibration Plot for the FESCN2+ Solutions
Step1:




First, 4 clean 25.00ml volumetric flasks were labeled #1-4.
Using pipettes, all the standard solutions were prepared as described in the
table.
5.00ml of the 0.200M iron nitrate solution was transferred to a 25.00ml
volumetric flask and the appropriate HSCN amount was added. This was
then diluted to the mark with 0.5M Nitric acid and mixed thoroughly.
The absorbance of all solutions was measured at 447nm and 0.5M HNO3
was used for the blank.
Determination of FESCN2+ at Equilibrium
Step1:




Another 4 clean volumetric flasks were labeled #1-4.
Using pipettes, the equilibrium solutions as described in the table in our lab
manual were prepared.
2.00X10-3M iron nitrate was transferred to a 10.00ml volumetric flask and
the appropriate amount of HSCN was added. This was then diluted to the
mark with HNO3.
Absorbance for all solutions was measures at 447nm.
Data/Calculations
Part1: Standard Solutions
Sol’n#
ml HSCN
A
1
2
3
4
0.50ml
1.00ml
1.80ml
2.50ml
0.168
0.318
0.597
0.794
Initial M
HSCN
0.00200
0.00400
0.00600
0.00800
Molarity
FeSCN2+
0.000040
0.000080
0.000144
0.000200
Part2 A: Equilibrium Solutions
Sol’n#
1
2
3
4
Ml HSCN
1.00ml
2.00ml
3.00ml
4.00ml
A
0.120
0.217
0.318
0.426
Molarity FeSCN2+
0.0000324
0.0000569
0.0000823
0.0001100
Equation attained from Calibration Chart: y=3969.7x+0.0088
𝑺𝒐𝒍′ 𝒏#𝟏 𝑴𝒆𝒒 𝑭𝒆𝑺𝑪𝑵𝟐+ =
𝑺𝒐𝒍′ 𝒏#𝟒 𝑴𝒆𝒒 𝑭𝒆𝑺𝑪𝑵𝟐+
𝟑𝟗𝟔𝟗.𝟕
= 𝟑. 𝟐𝟒 × 𝟏0−5
(𝟎. 𝟐𝟏𝟕 + 𝟎. 𝟎𝟎𝟖𝟖)
= 𝟓. 𝟔𝟗 × 𝟏𝟎−𝟓
𝟑𝟗𝟔𝟗. 𝟕
(𝟎. 𝟑𝟏𝟖 + 𝟎. 𝟎𝟎𝟖𝟖)
=
= 𝟖. 𝟐𝟑 × 𝟏𝟎−𝟓
𝟑𝟗𝟔𝟗. 𝟕
(𝟎. 𝟒𝟐𝟔 + 𝟎. 𝟎𝟎𝟖𝟖)
=
= 𝟏. 𝟏𝟎 × 𝟏𝟎−𝟒
𝟑𝟗𝟔𝟗. 𝟕
𝑺𝒐𝒍′ 𝒏#𝟐 𝑴𝒆𝒒 𝑭𝒆𝑺𝑪𝑵𝟐+ =
𝑺𝒐𝒍′ 𝒏#𝟑 𝑴𝒆𝒒 𝑭𝒆𝑺𝑪𝑵𝟐+
(𝟎.𝟏𝟐𝟎+𝟎.𝟎𝟎𝟖𝟖)
Part2 B: Equilibrium Constant Values
F𝒆𝟑+
Sol’n#
+
HSCN
FeSCN𝟐+
← − − −→
+ 𝑯+
Initial M 1.00x1𝟎−𝟑
2.00x1𝟎−𝟒
0.00
0.500
∆𝐌
-3.24x1𝟎−𝟓
-3.24x1𝟎−𝟓
+3.24x1𝟎−𝟓
0
Equil. M
9.68x1𝟎−𝟒
1.68x1𝟎−𝟒
3.24x1𝟎−𝟓
0.500
1
𝟎. 𝟓𝟎𝟎 × 𝟑. 𝟐𝟒𝒙𝟏𝟎−𝟓
𝑲𝒄 =
= 𝟗𝟗. 𝟔
𝟎. 𝟎𝟎𝟎𝟗𝟔𝟖 × 𝟎. 𝟎𝟎𝟎𝟏𝟔𝟖
F𝒆𝟑+
Sol’n#
+
Initial M 1.00x1𝟎−𝟑
FeSCN𝟐+
← − − −→
+ 𝑯+
2.00x1𝟎−𝟒
0.00
0.500
-5.69x1𝟎−𝟓
-5.69x1𝟎−𝟓
+5.69x1𝟎−𝟓
0
Equil. M 9.43x1𝟎−𝟒
1.43x1𝟎−𝟒
5.69x1𝟎−𝟓
0.500
∆𝐌
2
𝑲𝒄 =
F𝒆𝟑+
Sol’n#
𝟎. 𝟓𝟎𝟎 × 𝟓. 𝟔𝟗𝒙𝟏𝟎−𝟓
= 𝟐𝟏𝟏. 𝟎
𝟎. 𝟎𝟎𝟎𝟗𝟒𝟑 × 𝟎. 𝟎𝟎𝟎𝟏𝟒𝟑
+
Initial M 1.00x1𝟎−𝟑
HSCN
FeSCN𝟐+
← − − −→
+ 𝑯+
2.00x1𝟎−𝟒
0.00
0.500
-8.23x1𝟎−𝟓
-8.23x1𝟎−𝟓
+8.23x1𝟎−𝟓
0
Equil. M 9.18x1𝟎−𝟒
1.18x1𝟎−𝟒
8.23x1𝟎−𝟓
0.500
∆𝐌
3
𝑲𝒄 =
F𝒆𝟑+
Sol’n#
𝟎. 𝟓𝟎𝟎 × 𝟖. 𝟐𝟑𝒙𝟏𝟎−𝟓
= 𝟑𝟖𝟐. 𝟎
𝟎. 𝟎𝟎𝟎𝟗𝟏𝟑 × 𝟎. 𝟎𝟎𝟎𝟏𝟏𝟖
+
Initial M 1.00x1𝟎−𝟑
4
HSCN
HSCN
FeSCN𝟐+
← − − −→
+ 𝑯+
2.00x1𝟎−𝟒
0.00
0.500
-1.10x1𝟎−𝟒
-1.10x1𝟎−𝟒
+1.10x1𝟎−𝟒
0
Equil. M 8.90x1𝟎−𝟒
9.00x1𝟎−𝟓
1.10x1𝟎−𝟒
0.500
∆𝐌
𝟎. 𝟓𝟎𝟎 × 𝟏. 𝟏𝟎𝒙𝟏𝟎−𝟒
𝑲𝒄 =
= 𝟔𝟖𝟔. 𝟔
𝟎. 𝟎𝟎𝟎𝟖𝟗𝟎 × 𝟎. 𝟎𝟎𝟎𝟎𝟗
Mean Kc=
𝟗𝟗.𝟔+𝟐𝟐𝟏.𝟎+𝟑𝟖𝟐.𝟎+𝟔𝟖𝟔.𝟔
𝟒
= 347.3
RAD
347.3-99.6=247.7
347.3-211.0=136.3
347.3-382.0=34.7
327.3-686.6=359.3
Avg. Deviation
𝟐𝟒𝟕. 𝟕 + 𝟏𝟑𝟔. 𝟔 + 𝟑𝟒. 𝟕 + 𝟑𝟓𝟗. 𝟑
= 𝟏𝟗𝟒. 𝟔
𝟒
RAD
RAD=
𝟏𝟗𝟒.𝟔
𝟑𝟒𝟕.𝟑
= 𝟎. 𝟓𝟔 = 𝟓𝟔%
Discussion
When plotting the absorbance versus moles of FeSCN2+, the equation came out
to be y=3969.7x+0.0088 which seems accurate when comparing it to similar
experiments’ equations. From that equation, attaining the equilibrium moles for
FeSCN2+ was intuitive.
There is lots of room for error in terms of precision for this lab. Look at my rate of
deviation. When I was getting varying Kc values, I could tell something was amiss.
When transferring the HSCN, the amounts could’ve been mis-measured and that could
account for a different absorbances than needed, and thus affecting the equilibrium
constants and kc values simultaneously. Overall, this lab could’ve been executed in a
more accurate way myself, but now have a more optimal understanding of how to use
ICE tables to calculate the concentration at equilibrium for a specified solution.