Spectrophotometry Lab Report

Revised 6/15/17Name _________________________Station # ______ Date _____________________
CHM 101 Spectrophotometry
PRE-LAB
1. Calculate the molarities of diluted Copper (II) Sulfate solutions from the stock solution,
which is 0.1 M CuSO4.
Flask #
Volume of CuSO4
Solution (mL)
Volume of DI H2O
(mL)
Molarity (M) of
Solution
Cuvette #1
0
5
Blank (0.0 M)
Flask # 2
1
9
Flask #3
2
8
Flask #4
3
7
Flask #5
4
6
Flask #6
5
5
Flask #7
6
4
Flask #8
7
3
Flask #9
8
2
Flask #10
9
1
Cuvette#11
5
0
1.00 x 10-1
2. At what wavelength will the spectrophotometer be set?
3. The graph concentration vs. absorbance is associated with which Law?
Page 1 of 1
Revised 6/15/17
CHM 101 Spectrophotometry Experiment
Spectrophotometer wavelength set a 694nm.
Objective: to learn how to make dilutions from a given concentration (M1V1 = M2V2),
to learn how to plot a graph from data obtained and to gain some experience in the use of a
Spectrophotometer.
Background:
Spectrophotometric analysis is one method used to determine the concentration of a colored
substance in solution. Concentration is determined by calculating the amount of light that passes
through a sample and hits a detector. The more light that gets through, the less concentrated the
solution is (fewer molecules of the colored substance that absorb light).
A solution containing a colored substance absorbs a specific color (or colors) of visible light
and transmits the remaining colors. The amount of light a colored solution absorbs depends on
the concentration of the colored substance (mg/mL of solution). Another factor affecting the
amount of light absorbed is the color of light (wavelength in nm) selected.
A spectrophotometer is an instrument that measures the intensity of a light beam of a given
wavelength (color) that passes through a sample. Light of a given wavelength passes through
the sample, hits the detector, and the instrument can calculate the absorbance (A) of the
solution. If the appropriate wavelength is not selected, the sample will not absorb enough light
to make an accurate measurement.
Beer’s law states the absorbance is directly proportional to the concentration of a solution. If
you plot the absorbance (y-axis) versus the concentration (x- axis) you can determine the
concentration of an unknown solution using the graph or by using the equation for a line.
Beer’s Law is given by the equation
A = abC
in which, A = absorbance (measured with the Spectrophotometer)
a = constant (for a given substance at a given wavelength of light)
b = diameter of cuvette
C = concentration of absorber (CuSO4)
Safety Tips: Gloves, goggles and lab aprons must be worn. Please use caution.
Cuvettes must be transported to the spectrophotometer in a test tube rack.
Spectrophotometers are very expensive so please follow instructor guidelines.
Page 1 of 2
Revised 6/15/17
Experimental Procedure:
1. Each lab group will have an Erlenmeyer flask containing approximately 75mls of
0.1M CuSO4 on their lab table.
2. Label volumetric flasks #2-10 and cuvettes #1-11 using the wax pencil
provided.
3. Pipet desired amount of 0.1M CuSO4 solution in the labeled volumetric flask (see
Table 1 in Pre-lab). Then add de-ionized H2O from the squeeze bottle up to the line on
the volumetric flask. Cap the flask and invert at least “3” times to mix. Total
volume=10mls.
4. Transfer the solutions of CuSO4 from the volumetric flasks to the correct cuvette
(i.e. flask #2 to cuvette #2) and only fill the cuvettes half way.
5. For cuvette #1, fill it half way with de-ionized H2O and use it as the blank (to zero the
spectrophotometer); for cuvette #11, fill it half way with the 0.1M CuSO4 stock
solution.
6. Be sure to use a test tube rack to transport your cuvettes to the appropriate
Spectrophotometer (your Instructor will show you how to operate the
Spectrophotometer).
7. Use a clean Kimwipe to remove dirt, finger prints, etc. from each cuvette before
placing cuvettes in the sample holder of the Spectrophotometer.
8. At the conclusion of this experiment: be sure to remove cuvettes from the sample
holder, empty flask and cuvette contents into the waste container in fume hood “B”,
rinse flasks and cuvettes with water, use Windex cleaner to wipe down lab tables
and remember to wash your hands.
Page 2 of 2
Revised 6/15/17
Name _________________________Station # ______ Date _____________________
CHM 101 Spectrophotometry
Report Sheet
1. Absorbance of solutions of CuSO 4
Cuvette #
1
Absorbance Readings
Molarity (from pre-lab)
2
3
4
5
6
7
8
9
10
11
Page 1 of 3
Revised 6/15/17
2. Check Beer’s law by plotting absorbance vs. Molar concentration on the graph.
3. Is Beer’s Law valid for this system? Explain your answer.
Page 2 of 3
Revised 6/15/17
4. If Beer’s Law is valid, the value of “a” should be constant. Use A = abC or a = A/(bC)
and the data from the experiment to calculate values of “a” for the five solutions below.
The value of “b”, the diameter of the sample tube is normally 1 cm, so a = A/C in this
experiment.
Cuvette #
Calculated values for “a”
Average value for “a”
2
3
4
5
6
Show your calculations.
Page 3 of 3
Spectrophotometry
Make-Up Data
Instructions:
Use the following data to complete the data /calculation sheet of the experiment. Write your
weekly lab report for this lab, using this data and following the lab syllabus instructions.
1. Absorbance of solutions of CuSO 4
Cuvette #
1
Absorbance Readings
0.00
Molarity (from pre-lab)
0.00
2
0.020
0.01
3
0.042
0.02
4
0.065
0.03
5
0.09
0.04
6
0.104
0.05
7
0.13
0.06
8
0.145
0.07
9
0.165
0.08
10
0.188
0.09
11
0.208
0.10
Revised 6/15/17
Name _________________________Station # ______ Date _____________________
CHM 101 Spectrophotometry
Post Lab Questions
1. If the wavelength is changed, what will happen to the absorbance values?
2. A CuSO4 solution has an absorbance value of 0.512. Use your graph Concentration vs.
Absorbance, determine the concentration of this solution.
Page 1 of 1