# The Absorption Spectrum Of Cobalt Lab R

Topics: Concentration, Laboratory glassware, Absorbance Pages: 6 (456 words) Published: November 18, 2014
﻿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 range. Another error is when testing the absorbance, concentrations mixed up with other concentrations which give the reading either too high or too low. By add the low concentration first will lessen the chance of the error to occur. Questions:

1a. Dissolves 77.89 g of solid CoCl2 in to 250 ml of distilled water using an Erlenmeyer flask. 1b. Measure 250 ml of .150M CoCl2 using a beaker into a graduated cylinder then add 100 ml of distilled water. 1c. The maximum absorbance would be .483 since the date originally has .150 M of CoCl2. Conclusion:

The concentration decreases as the absorptivity decreases. This is true because when diluting CoCl2 with water, the concentration of the CoCl2 decreases due to the amount of CoCl2 that was originally in the tube decrease.

Thang Dinh11/17/2014

Appendix:
Page 3. Figure 1.1 Table of Absorption Spectrum of Cobalt(II) Chloride
Figure 1.2 Graph of Absorption Spectrum of Cobalt (II) Chloride Page 4. Figure 1.3 Table of Absorption Spectrum of Unknown B.