IA lab work on spectrophotometric determianation of Kc
of the Iron(3)Thiocyanate system
Since I will not change the temperature in my experiment, the equalibrium constant will stay
the same, and therefore I can measure the light absorbance and use the Lambert Beer-Law to
find the initial concentartion of each species in the system and then calculate the Kc.
X( initial concentration of Iron3 and Thiocyanate)=A(absorbance of Iron(3)Thiocyanate solution )/E*l
Equipment:
-4 test tubes with 4 corks
-test tube rack
-10 ml pipitte with pump thing
Solutions:
-20 ml of Iron (3) solution
-20 ml of Thiocyanate solution
-20 ml of water H20
Samples
The amount of
Thiocyanate
solution
SCN-
The amount of
water
H20
Iron 3 solution
Fe3+
1
2
3
4
2 ml
4 ml
5 ml
9 ml
8 ml
6 ml
5 ml
1 ml
5 ml
5 ml
5 ml
5 ml
Sample
1st trial (in
2nd trial (in 3rd trial (in
4th trial (in
5th trial
nanometers of
the molar
absorptivity)
nanometers of
the molar
absorptivity)
nanometers of
the molar
absorptivity)
(in
nanometers of
the molar
absorptivity)
nanometers of
the molar
absorptivity)
1 (2 ml of
Thiocyanate
solution)
2 (4 ml of
Thiocyanate
solution)
3 (6 ml of
Thiocyanate
solution)
4 (10 ml of
Thiocyanate
solution)
Procedure:
1)
2)
3)
4)
5)
6)
Label the test tubes with numbers: 1,2,3,4.
Using a pipette, add 5 ml of Iron(3) solution to each test tube.
For sample 1, add corresponding amount of water to the 1st test tube.
For sample 1, add corresponding amount of Thiocyanate solution to the 1st test tube.
Mix it and leave it for 3-4 minutes to reach the equalibrium.
Transfer the prepared solution from the 1st tube to the the special small test tube from
the spectometer.
7) Put in into spectometer and record the absorbence of light.
8) Repeat step 1-6 with another samples.
9) Do 3 trials for each sample.