Name: Nathania Njotodjojo
CHEM 121, sec. 4368
Date : 07/19/2011
Project Werner: Characterization of Cobaltammine Complex
Reference: Project Werner: Characterization of Cobaltammine Complex, pages 12-19.
Summary Abstract:
The goal of the project was to determine the identity of an unknown Cobaltammine
complex of the form [Co(NH3)nL(6-n)]Yq by using a series of procedures. The procedures are
divided into four parts: first by determining the molar mass using beer’s law, followed by
titration to determine the number of ammonia ligands, then measuring the electrical
conductivity in solution to determine the number of ions, and finally generating a visible
spectrum to determine the wavelength of maximum absorbance in which it would depend
on the ligands it contains. Combining all the results for each part, the possible
Cobaltammine complex of the compound is [Co(NH3)5Cl]Cl2.
Results:
Part A: Determination of Molar Mass Using Beer’s Law
The analysis begins by reacting 0.1454 g of the Co3+ compound with oxalic acid:
2 Co3+ + H2C2O4 (aq)  Co2+ (aq) + 2 CO2 (g) + 2 H+ (aq). Next, the Co2+ was placed in an
environment rich in thiocyanate to push the following equilibrium essentially to the right:
[Co(OH2)6]2+ (aq) + 4 SCN- (aq)  [Co(SCN)4]2- (aq) + 6 H2O (l). The blue [Co(SCN)4]2- ion was
then quantified using spectrophotometry and Beer’s Law.
A blank and 4 different concentrations of [Co(SCN)4]2- prepared by dilutions of stock
solution of 0.0003013M [Co(SCN)4]2- were used as the standards to generate the Beer’s
Law plot. The absorbance for each standard and the unknown was measured using
Spectronic 20D spectrophotometer that was set to 625nm. From the data, concentration vs
absorbance graph can be plotted and using Excel program, the equation of the best-fit line
was: y=1669x. The absorbance of the compound was 0.286 and by inputting the value to
the equation, the concentration can be calculated. After that, the number of moles can be
calculated and finally the molar mass of the complex can be determined and it is equal to
353.87 g/mol.
Part B: Titration of Ammonia Ligands
The number of ammonia ligands present in the compound can be determined using
partial titration. Firstly, the cobalt complex was decomposed by heating it in NaOH (aq):
Cobalt (III) complex + OH- (aq)  Cobalt oxides + x NH3 (g). The NH3 gas would then reacted
with a known quantity of cold HCl solution: H+ (aq) + NH3 (g)  NH4+ (aq). The amount of
unreacted HCl was measured by titrating the solution with standard 0.1060M NaOH using
bromocresol green as the appropriate indicator. The difference between the original
amount of pure HCl and the amount of HCl titrated is equal to the amount of HCl that
reacted with ammonia. Therefore the mol of ammonia (equals to mol of HCl) is 2.1733 x
10-3 mol. The number of mol of the compound can be calculated by dividing the mass of the
compound used by the molar mass calculated in part A. Dividing the number of mol of NH 3
with the number of mol of the cobalt (III) compound gave the number of ammonia ligands
present in the complex and it is equal to 5.
Part C: Measurement of Electrical Conductivity in Solution
The conductivities measured are 270, 275, and 277 μS/cm, giving an average
conductivity of 274 μS/cm. The molar conductivity can then be calculated by using the
formula M= (1000 cm3/L x C)/M. To calculate the molarity, M= (mass of complex
used/molar mass calculated from part A)/volume of deionized water used for dilution. The
molar conductivity can then be calculated and is equal to 274 S cm2/mol. The value of
molar conductivity of 274 S cm2/mol corresponds to 3 ions of the complex (fit within the
range of molar conductivity of 230- 280 S cm2/mol).
Part D: Visible Spectrum of Cobaltammine Complex
The wavelength of light absorbed by the cobalt ammine complex depends on the
ligands it contains. The absorbance is measured for each wavelength in the range of 400nm
to 700nm with 10nm interval using the Spectronic 20D spectrophotometer. The
wavelength in which the absorbance measured is the maximum is 530nm. The result
corresponds to the complex [Co(NH3)5Cl]Cl2.
Conclusion:
From part A, the molar mass was determined and it was equal to 353.87 g/mol. In
the next part, the number of ammonia ligands present in the compound was found to be 5.
In part C, molar conductivity of 274 S cm2/mol corresponds to 3 ions of the complex.
Finally in the visible spectrum, it was found that maximum wavelength of 530nm
corresponds to the complex [Co(NH3)5Cl]Cl2. Combining all the results from each part, it can
be concluded that the most possible identity of the complex is [Co(NH3)5Cl]Cl2.
2. Postlab Questions
1. The standards used to generate the Beer’s Law plot are in the form of blue [Co(SCN)4]2.
Therefore, the magenta Co3+ compound should also be converted to blue [Co(SCN)4]2- so
that the absorbance can be compared and the concentration can be calculated.
2. The spectrophotometer was set to 625nm since the color of the [Co(SCN)4]2- standards
are blue. The complementary color for blue is orange in which its wavelength interval is
590-635nm. Therefore wavelength of 625nm was used since it would have the highest
absorbance and thus it would be easier to generate the Beer’s Law plot.
3. Beer’s Law plot on the next pages.
4. Absorbance of the sample= 0.286
Equation of best-fit line: y=1669x
x (concentration)= 0.286/1669= 1.7136 x 10-4 M
mol of complex= 1.7136 x 10-4 M x 2.5L= 4.284 x 10-4 mol
MM= 0.1516 g/ mol= 353.87 g/mol
5.
Since the solution being titrated contains both unreacted HCl (aq) and NH4+ (aq), the first
equivalence point is reached when all of the HCl has been neutralized and the second
equivalence point is reached when all of the NH4+ has been neutralized. Since the amount of
unreacted HCl but not the NH4+ ions was the one needed to be known, the titration was
stopped at the first equivalence point by using bromocresol green (pKa=4.6) as the
appropriate indicator.
6. Refer to yellow pages for detailed calculations
mol of reacted HCl= mol of NH3= 2.1733 x 10-3 moles
7. mol of Co(III) complex= 0.1454g/353.87 g/mol= 4.1088 x 10-4 moles
number of NH3 ligands= mol of NH3/mol of Co (III) complex= 5 ligands.
8. M= (1000 cm3/L x C)/M
M = 1000 cm3/L x [(270+275+277) μS/cm / 3] = 274 S cm2/mol
(0.0884 g / 353.87 g/mol) / 250mL
The number of ions per formula unit= 3 ions.
9. The maximum wavelength is 530nm. The color of the complex observed as a compound
and when it is diluted in deionized water is magenta. The color observed is the
complementary color of the color of the highest absorbance. The wavelength 530nm
corresponds to green color. Green (the color with the wavelength of maximum absorbance)
is the complementary color of magenta (the color observed by eyes).
3. Beer’s Law Plot
Concentration of [Co(SCN)4]2- vs Absorbance
0.45
0.4
Absorbance
0.35
y = 1669x
R² = 0.9941
0.3
0.25
0.2
0.15
0.1
0.05
0
0
0.00005
0.0001
0.00015
0.0002
Concentration (M)
0.00025
0.0003
4. Visible Spectrum of Cobaltammine Complex
Wavelength (nm) vs Absorbance
0.6
Absorbance
0.5
0.4
0.3
0.2
0.1
0
400
450
500
550
600
Wavelength (nm)
650
700