Data:
Standard Solutions for the Beer’s Law Plot
a. Fe(NO3)3 in 0.1 M HNO3 concentration = .2 M
b. NaSCN in 0.1 M HNO3 concentration = .002 M
c. reaction temperature = 25 Celcius
Table 1 Reference
0 Vol. of
NaSCN
added (mL)
Initial SCN 0
ion conc.
M
Equilibriu
0
m FeNCS
ion conc M
Abs at
0
447nm Std.1
.5 Std.2
1 Std. 3
1.50 Std.4
2 Std.5
2.50 4 x 10-5 8 x 10--5 1.2 x 10-4 1.6 x 10-4 2 x 10-4 4 x 10-5 8 x 10-5 1.2 x 10-4 1.6 x 10-4 2 x 10-4 .045 .068 .154 .287 .317 Beer’s Law Plot BEER'S LAW PLOT
0.35
0.3 f(x) = 0.07x - 0.09 0.25
0.2
Abs at 447nm 0.15
0.1
0.05
0
0.0 5 . 0 E- 5 8 . 0 E- 5 0 . 0 0 12 0 . 0 0 16 0.0002 Equilibrium FeNCS ion conc M Table 2
Equilibrium Solutions to Determine Equilibrium Constant for Reaction
a. Fe(NO3)3 in 0.1 M HNO3 concentration = .002 M
b. NaSCN in 0.1 M HNO3 concentration = .002 M
c. reaction temperature = 25 Celcius
1 Fe(NO3)3 in
0.1 M
HNO3
added
Vol. of
NaSCN
added (mL)
Initial Fe3+
ion conc.
(M)
Initial SCN
ion conc.
Abs. at
447nm
Equil.
FeNCS2+ ion
conc. (M) Equil. 1
1 Equil. 2
1 Equil. 3
1 Equil. 4
1 Equil. 5
1 Equil. 6
1 0 .2 .4 .6 .8 1 .001 .001 .001 .001 .001 .001 0 2 x 10-4 4 x 10-4 6 x 10-4 8 x 10-4 .001 0 .257 .360 .383 .493 .556 1.38 5.13 6.64 6.97 8.58 9.496 1.379 5.129 6.639 6.969 8.579 9.495 1.379 5.129 6.639 6.969 8.579 9.495 0 4.2 3.84 3.63 3.4 3.25 3+ Equil. Fe
ion conc.
(M)
Equil. SCN
ion conc
(M)
Equil.
Constant,
Keq
Mean Keq 3.05 1. Briefly explain why the concentration of ferric nitrate was 100-fold higher when the solutions for
the Beer’s law plot were prepared (Table 1) vs. the equilibrium solutions of Table 2. 2 2. Briefly explain what effect (if any) each of the following errors would have on the indicated
parameter. Indicate if the new value would be increased, decreased, or remain the same. The cuvette for the Std 4 solution was accidentally inserted the wrong way into the cuvette
holder and the light path was reduced from 1 cm to 0.5 cm.
o What effect would this have on the recorded absorbance of Std. 4?
The effect that it would have would be that the absorbance value would
decrease. o The mistake was realized after the solutions had already been discarded. Your lab
partner Bob suggested just eliminating the S4 data point from the Beer’s Law plot to
make it more accurate since you didn’t really need it anyway. Do you agree with
Bob?
No, the S4 data point would make a difference and it should be done again to
have accurate results. The volumes of Fe(NO3)3 and NaSCN were mistakenly reduced by 5% when preparing the
standard solutions for the Beer’s Law plot but the total solution volumes remained the
same. How would this affect each of the following? Assume that the error is known and
calculations can be adjusted if possible.
o The calculated initial SCN- ion concentration 3. The equation for a Beer’s Law plot is y = 2012.5x + 0.003. If the recorded absorbance for an
equilibrium mixture is 0.362, what is the concentration of FeCNS 2+ (aq)? 3 4. You‘re preparing the solutions for this lab and realize that you don’t have any sodium
thiocyanate but you do have some potassium thiocyanate. Can you substitute the potassium
thiocyanate for the sodium thiocyanate without any problems? Briefly explain your answer. 4

 

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