Report Form: The Value of an Equilibrium Constant by
Complex Ion Formation
(Total 80 Points)
Note: In preparing this report you are free to use references and consult with
others. However, you may not copy from other students’ work (including your
laboratory partner) or misrepresent your own data (see honor code).
Name(Print then sign): ___________________________________________________
Lab Partner: ____________________________________________________________
1. Calibration Curve
Concentration _________ _________ _________ _________ _________
Absorbance
(430 nm)
_________ _________ _________ _________ _________
Absorbance
(  max )
_________ _________ _________ _________ _________
Make 2 plots of absorbance vs. concentration in order to find your value of molar
absorptivity for the spectrophotometer that you used. Attach your plots to the back of this
report form.
 max : _____________________________________
Peak Molar Absorptivity: ____________________
Molar Absorptivity at 430 nm: ________________
Correlation Coefficient of each ( R 2 ): __________________
__________________
2. Data
TestTube #
1
2
3
4
5
Reagents (mL)
1
2
3
4
5
Fe(NO3)3
5.00
5.00
5.00
5.00
5.00
KSCN
1.00
2.00
3.00
4.00
5.00
0.5 M HNO3
4.00
3.00
2.00
1.00
0.00
Absorbance
3. Calculations
A. Calculation of K c assuming the reaction
Fe3 (aq)  SCN  (aq)  FeSCN 2 (aq)
1. Find the initial number of moles of Fe 3 and SCN  in the mixtures in test tubes 1
through 5.
2. Using Beer’s law, and your calibration curve, enter the experimentally determined
value of [ FeSCN 2 ] at equilibrium for each of the mixtures in the next to last
column in the table.
3. Find the number of moles of FeSCN 2 in each of the mixtures, and enter the
values in the fifth column of the table. Note that this is also the number of moles
of Fe 3 and SCN  that were used up the reaction.
4. From the number of moles of Fe 3 and SCN  initially present in each mixture,
and the number of moles of Fe 3 and SCN  used up in forming FeSCN 2 ,
calculate the number of moles of Fe 3 and SCN  that remain in each mixture at
equilibrium. Enter the results in columns 3 and 4 of the table.
5. Find the concentrations of all of the species at equilibrium. The volume of the
mixture is 10.00 mL, or 0.0100 liter in all cases. Enter the values in columns 6
and 7 of the table.
6. Calculate K c for the reaction for each of the mixtures by substituting values for
the equilibrium concentrations of Fe 3 , SCN  , and FeSCN 2 in the equilibrium
expression.
Mixture
Initial #
Moles
Fe3+
Initial #
Moles
Equil #
Moles
Equil #
Moles
SCN-
Fe3+
SCN-
Equil #
Moles
FeSCN2+
Equil.
Conc.
Fe3+
Equil.
Conc.
Equil
Conc.
SCN
FeSCN2+
1
2
3
4
5
On the basis of your results, what can you conclude about the validity of the equilibrium
concept, as exemplified by Equation 2?
What do you conclude about the formula of the iron (III) thiocyanate complex ion?
Kc