AP Chemistry Investigation 1 Page 1

Investigation 1: What Is the Relationship Between the Concentration of a

Solution and the Amount of Transmitted Light Through the Solution?

Central Challenge
How can light be used to study color and determine concentrations of chemical species in solutions?

Context for This Investigation
Measuring 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 chemistry laboratory. The basic principles of spectral analysis that students learn in
school can also be applied to the more sophisticated instrumentation required to access the ultraviolet,
infrared, and X-ray regions. What students learn by performing the lab will help them to understand more
sophisticated instruments that they may encounter at a college or university.

In a visible spectrophotometer, students shine a beam of light into a solution containing the sample, and
detect how much of it comes out of the other side of the solution. By comparing the amount of light
transmitted by the pure solvent to the amount transmitted when the sample is dissolved in it, we can
calculate a quantity called the absorbance. Spectrophotometers can report measurements as percent
transmittance (%T) or directly as absorbance. In this investigation, students will be guided to discover the
relationship between transmittance and concentration and ultimately the relationship between
transmittance, absorbance, and concentration of a solution.

Materials
spectrophotometer
cuvettes
pipettes
stock solution of blue liquid food dye
blue-colored sports drink
distilled water
test tubes (20 mL)
graduated cylinders (10 mL)
test tubes
test tube rack
Safety and Disposal
All of the food dyes can be flushed down the sink with plenty of water.

Prelab Guiding Questions
Step 1: A stock solution of the blue #1 (Brilliant Blue) molecule dissolved in water with a known
concentration..




Figure 1. FD&C blue #1 (Brilliant Blue) molecule

Step 2: Explore how light, transmitted by a solution, is related to the concentration of the solution. The
appropriate wavelength to take transmittance measurements using the SPEC 20 for this dye is 630 nm.
Step 3: Students work in groups of 3 or 4 use the following dilutions of stock/water:
Group 1: %transmittance data for the following dilutions: 10 mL stock/0 mL water, 8 mL stock /2 mL
water
Group 2: %transmittance data for the following dilutions: 6 mL stock /4 mL water, 4 mL stock /6 mL
water
AP Chemistry Investigation 1 Page 2

Group 3: %transmittance data for the following dilutions: 2 mL stock /8 mL water, 0 mL stock /10mL
water
Group 4: % transmittance data for the following dilutions: 3 mL/ stock 7 mL water, 1 mL/ stock 9 mL
water

Step 4: Have student groups record the % transmittance of their diluted solutions of known concentration
using the SPEC 20 (or other spectrophotometer) in a data table in a central location in the lab.

Table 1. %Transmittance of diluted solutions of known concentration
Solution Dilution Ratio mL
stock/mL Water
Molar
Concentration(M)
Measured
%Transmittance
Measured
Transmittance as a
Decimal
1. (stock solution) 10
mL/0 mL

2. 8 mL/2 mL
3. 6 mL/4 mL
4. 4 mL/6 mL
5. 3 mL/7mL
6. 2 mL/8 mL
7. 1 mL/9 mL
8. 0 mL/10 mL

Step 5: Determine the relationship between transmittance and molarity of the solution. Graph the data in the
data table (plot M on the x-axis versus transmittance as a decimal on the y-axis).

Step 6: What mathematical routine could we choose to plot to get a linear relationship between
transmittance and molarity that has a positive slope, and which line goes through zero? Try out and report
back to the whole group: (A) 1/T versus [dye], (B) 1 10T versus [dye], (C) logT versus [dye], (D) logT
versus [dye]

Investigation: Many common sports drinks contain blue #1 dye. You will use the relationship between
transmittance, absorbance, and concentration (as well as their calibration line from the prelab) to determine
the concentration of this dye in the sports drink.

Procedure: Obtain a sample of the blue-colored sports drink. Write out your procedure in your
notebook.

Data Collection and Computation in Your Notebook
1. Determine the molar concentration of blue #1 dye in the sports drink. Show all work.

2. Determine the mass of blue #1 dye found in 500 mL of the drink. Show all work.

Argumentation and Documentation
In the conclusion of lab, have students justify the procedure they chose, the instrumentation they used,
and the selection of the kind of data they decided was needed to determine the concentration of blue
#1 dye in a blue-colored sports drink containing only this dye.

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Postlab Assessment
1. 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 3.5 10
6
M. What was the concentration of the
original stock solution before dilution?

2. If a 0.10 M solution of a colored substance has a maximum absorbance at 500 nm and an absorbance of
0.26 M at this wavelength, what will be the measured absorbance of a 0.20 M solution at 500 nm?

3. The spectrophotometer really measures the percent of light that is transmitted through the solution. The
instrument then converts %T (transmittance) 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?

Explanation to Strengthen Student Understanding

Transmittance and Absorbance Relationship
The relationship between transmittance and absorbance is Abs = log10 (T) where T is the transmittance,
rather than the %T (i.e., 0.50, not 50%). When is the absorbance is 1.0, only 10% of the light beam is
reaching the detector. When Abs = 2, only 1% of the light beam is reaching the detector and when Abs = 3,
only 0.1% of the light is reaching the detector. It should be no surprise that the accuracy and sensitivity of
low-cost instruments start to suffer at absorbance values higher than 1.5. If a sample shows an absorbance
higher than 1.5, it is a good idea to dilute the sample by a factor of 5 or 10 and re-measure. The extra work
is paid back in better accuracy in your measurement. A useful rule is dont accept measurements of
>1.5Abs. If you get such an absorbency value, dilute the sample and measure it again.