# The Absorption Spectrum Of Cobalt Lab R

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The Absorption Spectrum of Cobalt(II) Chloride
Thang Dinh
Lab partner: Alex Yang
CHM 151 – 102
11/17/2014

Introduction:
Many compounds absorb a certain spectrum of visible light. Since Beer’s Law describe the relationship between molar absorptivity and concentration, one could use his equation A = kc to find the unknown concentration by the known absorptivity. The graph of absorption spectrum is represented by a linear regression. From the graph, one could deduct that as the concentration decreases, the absorptivity also decreases.
Materials and Methods:
Obtain 7 test tubes and labeled them. Each test tube, pipet in CoCl2 decreasing order with increment of 1 ml. Then pipet in the distilled water in increasing order also with increment of 1 ml. Measure the spectrum with the colorimeter.

Results:
Figure 1.1 Table of Absorption Spectrum of Cobalt(II) Chloride
Test Tube No.
A
C
1
.483
.150 M
2
.374
.120 M
3
.346
.105 M
4
.289
.09 M
5
.234
.075 M
6
.182
.060 M
7
.075
.03 M

Figure 1.2 Graph of Absorption Spectrum of Cobalt (II) Chloride

Slope : k = 3.37 L/ mol
Figure 1.3 Table of Absorption Spectrum of Unknown B ml of unknown ml of distilled water
A
4.0
4.5
.402
3.0
5.5
.247
2.0
6.5
.097
Final Determination

Volume of unknown (mL)
4.0 ml
Volume of distilled water (mL)
4.5
A
.402
C
.119 M
Undiluted solution:
.253 M
Calculation:
A = kc
.402 / 3.37 (L/mol) = c = .119 M
M1V1 = M2V2
(.119 M) (8.5 mL) / (4.0 mL) = M2 = .253 M
Discussion:
The color of the unknown B from was much darker than the known CoCl2 which mean that the concentration is much higher than the known solution. The result of the undiluted solution is .253 M which is higher than .150 M. So, the concentration of the diluted solution is accurate because it supposes to be higher due to the difference in color. A systematic error occurs in this experiment is when pouring out the solution from each test tube, some residues stick on the wall of the test tubes which make the value not in the desired

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