Lab: Determining Molarity Through AbsorbanceThis is a featured page

The concentration of a solution also changes the appearance of the solution. For example, a more concentrated solution of CuSO4 will appear to be more dense than a much more diluted solution of CuSO4. This means that there is a relationship between the concentration of a solution and its molarity. Thus, this experiment is set up to find the type of relationship between the molarity and absorbance of CuSO4 and be able to use this data to estimate molarities of unknown solutions based on a given absorbance rate.

If the concentration and the absorbance rate of a particular solution is tested, then the resulting relationship is most likely linear.

- Computer with Logger Pro application
- Colorimeter
- 5 Test Tubes
- 2 x 10 mL graduated cylinders
- 2 x 100 mL beaker
- 0.4M CuSO4 solution
- Graduated pipette
- Distilled water
- Test tube Rack
- Stirring Rod

1) Obtain 30 mL of
CuSO4 in a 100 mL beaker. Label the 5 test tubes 1, 2, 3, 4 and 5 respectively and put them in the test tube rack. Put the following liquids into each test tube and stir with a stirring rod:

Test Tube Number 0.40 M CuSO4 (mL) Distilled Water (mL)
1 2 8
2 4 6
3 6 4
4 8 2
5 10 0

2) Connect a colorimeter to the Lab Pro set. Connect the Lab Pro set to the computer. Open up the Logger Pro application, and open the file "17 Colorimeter". Now, calibrate your colorimeter by preparing a cuvette filled with 3/4 water. Place the cuvette into the colorimeter slot and close the lid. Set the wavelength knob to the "0% T" position and wait for the voltage reading to stabilize. When the voltage reading has stabilized, type "0" into the first edit box and press "Keep". Now turn the wavelength knob to 635 nm. Wait for the voltage reading to stabilize then type in "100" into the second edit box and press "Keep". Your Logger Pro and colorimeter should be reading to collect data. Your setup should look like this:

Figure 1: Lab setup
Lab: Determining Molarity Through Absorbance - Science Learning Resources
3) Press "Collect" on the Logger Pro to begin quite awesome collecting data from the colorimeter. Fill an empty cuvette 3/4 full with solution from test tube 1. Now place the cuvette into the colorimeter slot and watch the absorbance reading (Y-value) on the logger pro stabilize. Once stable, click "keep" and type the molarity of the solution (ex. "0.08" for test tube 1) into the edit box. Once the data is plotted, empty and thoroughly rinse the cuvette.

4) Repeat step 3 with solutions from test tube 2, 3, 4 and 5. Once testing is finished, a linear plot graph should form. Click the linear regression button to create an estimated equation modeling the relationship between the absorbance and concentration of CuSO4.

Data Collection And Analysis

Table 1: Collected data of CuSO4 absorbance at different concentrations

Concentration (mol/L)

The collected datapoints can be modeled by a single linear equation, which can be then used to determine solutions with unknown concentrations. The linear regression yields the equation y = 0.1087x + 0.0847, where x is concentration and y is absorbance.

An solution of
CuSO4 with an unknown concentration was presented. To find out the concentration of the solution, the absorbance of the solution is required. By placing a sample of the solution into the colorimeter, the absorbance can be found. Using the equation obtained using linear regression, the absorbance value can be plugged in to directly find the corresponding concentration.

Graph 1

The unknown solution's absorbance was 0.29

0.29 = 1.35875x + 0.0847
0.2053 = 1.35875x
x = 0.1511 mol/L

Concentration of the unknown solution = 0.1511 M

The molar absorptivity of CuSO4 can also be found by using the equation: A = e*b*c, where 'e' is the molar absorptivity, 'b' is how much distance the light source passes through the solution in centermeters, 'c' is the concentration of the solution in mol/L. Using a data point from table 1,

0.176 = e * 1 cm * 0.08M
0.176 / 0.08M = e
2.2 L / (mol * cm) = e

Thus, we can use the equation A = 2.2 L / (mol * cm) * 1 cm * C to model the absorbance of CuSO4 at different concentrations.
This equation also models the linear relationship between concentration and absorbency. According to this derived equation above, for every 1 mol/L increase in concentration of CuSO4, there will be a 2.2 increase in absorbance.

The relationship between absorbance and molarity of CuSO4 is linear. The linear regression shows that for every 1 mol/L increase in concentration, the absorbance increases by 1.35875. This shows that my hypothesis was correct about how absorbance and concentration have a direct relationship.

- Measurement Errors
Due to the nature of measuring liquids, our concentrations cannot be 100% accurate. In effect, the resulting absorbances of the CuSO4 solutions can be slightly off due to the inaccurate concentrations.

- Cuvette Errors
The cuvette itself supposedly has a stable and set transmittancy. However, previous use of the cuvette could lead to stains and possibly varying the transmittancy. As a result, the transmittancy may yield a slightly inaccurate reading of the linear regression.

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