WHAT IS THE RELATIONSHIP BETWEEN THE CONCENTRATION OF A SOLUTION AND THE
AMOUNT OF LIGHT TRANSMITTED/ABSORBED THROUGH THE SOLUTION?
CENTRAL CHALLENGE:
 How can light be used to study color and determine concentrations of chemical species in solutions?
o Measureing how much of which wavelengths of light are absorbed by a substance, and getting
useful information about that substance from the results, is the scientific discipline of spectroscopy.
The visible spectrum is the only part of the electromagnetic spectrum that we can access with the
equipment found in a typical high school laboratory. In a visible spectrophotometer, a beam of light
is shown into a solution containing the sample, and a detector measures how much of it comes out
– transmittance, or doesn’t come out – absorbance. By comparing the amount of light transmitted
by the pure solvent to the amount transmitted by the solution, we can calculate absorbance. This
can be used to determine concentration of unknown solutions.
PRELAB DISCUSSION / QUESITONS:
1. You are being given sample of a stock solution of of the blue #1 (Brilliant Blue) molecule dissolved in water with
a known concentration of: ___________ M (_______ μM). Shown Below
2. In the first part you will be exploring how light, transmitted by a solution, is related to the concentration of the
solution. For this experiment you will be setting the spectrophotometer at a wavelength of 630 nm. Why?
3. Each group will be responsible for collecting the percent transmittance of the molecule in solution at two
concentrations. The values will be reported into the Google Spreadsheet.
 Period 1 Link
 Period 3 Link
 Period 9 Link
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Group 1: Solution 1 & 2 (Reported value will be an average with group 6)
Group 2: Solution 3 & 4 (Reported value will be an average with group 7)
Group 3: Solution 5 & 6 (Reported value will be an average with group 8)
Group 4: Solution 7 & 8 (Reported value will be an average with group 9)
Group 5: Solution 9 & 10 (Reported value will be an average with group 10)
Group 6: Solution 1 & 2 (Reported value will be an average with group 1)
Group 7: Solution 3 & 4 (Reported value will be an average with group 2)
Group 8: Solution 5 & 6 (Reported value will be an average with group 3)
Group 9: Solution 7 & 8 (Reported value will be an average with group 4)
Group 10: Solution 9 & 10 (Reported value will be an average with group 5)
4. Determine the relationship between transmittance and Molarity of the solution using Microsoft Excel (You may
have to access this through Citrix and will just have to transfer data from the Google file.) Plot Concentration
(in micromolar) on the x-axis and Transmittance on the y-axis.
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This is not a linear relationship. Linear relationships are typically more useful in science. How could we
make this a linear relationship with a positive slope that goes through (0,0)?
This graph will be used to determine the concentration of your unknown solution. This is possible using the
equation of a straight line: y = mx + b.
o What is y, m, x, b?
5. Absorbance is related to transmittance through the following relationship: Abs = - log(T). Compare your
measured absorbance to a calculated absorbance.
 Because of this:
o when you have an absorbance of 1: 10% of the light beam is reaching the detector
o when you have an absorbance of 2: 1% of the light beam is reaching the detector
o when you have an absorbance of 3: 0.1% of the light beam is reaching the detector.
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You can see that low cost instruments tend to become fairly inaccurate with absorbance values of 1.5 or
greater. Therefore using dilute solutions is helpful.
INVESTIGATION:
1. You will now use the spectrophotometer to perform a quantitative analysis on a blue sports drink to find out
how much blue #1 die is in the drink.
a. Using you blue colored sports drink determine the molarity or concentration of blue #1 die in your
sports drink. Show all work.
b. Determine the mass of blue #1 dye found in 500 mL of the drink. Show all work.
POSTLAB ASSESSMENT:
1. Show all work for the investigation part.
2. Suppose a solution was too concentrated for an accurate reading with the spectrophotometer. The
concentrated solution was diluted by placing 1.00 mL of the concentrated solution in 4.00 mL of water. The
solution was then placed in the spectrophotometer and an absorbance was obtained and after a few
calculations the molar concentration was calculated to be 3.5x10-6 M. What was the concentration of the
original stock solution before dilution?
3. If a 0.10 M solution of a colored substance has a maximum absorbance at 500 nm and an absorbance of 0.26 at
this wavelength, what will be the measured absorbance of a 0.20 M solution at 500 nm?
4. The spectrophotometer really measures the percent of light that is transmitted through the solution. The
instrument then converts %T into absorbance by using the equation you determined in the prelab section. If the
absorbance of a sample is 0.85, what is the percent of light transmitted through the colored sample at this
collected wavelength?