Determining the Concentration (Molarity) of Blue Dye in Gatorade
Introduction:
We have seen how colors appear to our eyes as a result of white light falling on those
colors and the colored object absorbing all of the colors from the white light EXCEPT
those of the color. For example, a blue dye appears blue because white light falls upon
the blue dye and the dye absorbs ALL colors except blue. Blue is then reflected into our
eyes.
A Spectrophotometer is a device that measures the amount of light of a particular
wavelength (color) that comes through a solution. Light of all wavelengths is shined on
the sample, and the spectrophotometer is set at a particular wavelength to detect if that
wavelength passes through the sample.
We are going to make several solutions of blue dye (which has a wavelength of
approximately 480-430 nanometers) and pass a white light through it to see how much
ORANGE wavelength (yes, ORANGE) gets through at 630 nm.
Why do you think we choose the range of 630 nm to detect when blue dye is actually
in the range of 480-430?_______________________________________
Solution Preparation:
The data table below tells you how to prepare your eight-samples from the stock solution
of 1 Molar (1 mole/liter) Blue Dye.
Solution #
1
2
3
4
5
6
7
8
Dilution Ratio
mL Stock/mL
H2O
0 mL/10 mL
1 mL/ 9 mL
2 mL/ 8 mL
3 mL/ 7 mL
4 mL/ 6 mL
6 mL/ 4 mL
8 mL/ 2 mL
10 mL/ 0 mL
Molar
Concentration
(Moles/Liter)
0M
Percent
Transmittance
(%T)
1/decimal
Percent
form of
Transmittance percent
in Decimal
transmittance
form
(1/T)
1M
Your first solution sample will be pure water. Your second will be 1 mL of stock
solution and 9 mL of pure water….etc.
How to determine the Molar Concentration (Moles/Liter):
You will notice that the molar concentration is filled in for you for solutions #1 and
#8. Obviously, Solution #1 is a pure water that has no molarity of blue dye
(0 Moles/Liter). Solution #8 is pure Blue Dye and so it has a concentration of 1 M.
To determine the others, you need to use the formula: M1V1 = M2V2
M1 = molarity of stock solution (always 1 M in each solution. That’s the stock molarity)
V1 = Volume amount (mL) of stock solution to be added (varies)
M2 = Resulting molarity (Molar Concentration in Moles/Liter)
V2 = Volume amount (mL) of resulting solution (always 10 mL in each solution)
Obviously, you have three of the four variables available to you and you need to solve
for the fourth (Molar Concentration).
Show your work below for each of the Molar Concentrations for solutions 2-7
Solution 2________________________________________________________
Solution 3________________________________________________________
Solution 4________________________________________________________
Solution 5________________________________________________________
Solution 6________________________________________________________
Solution 7________________________________________________________
How to determine the Percent Transmittance:
Once you have your samples made, take them to the spectrophotometer and carefully
put each sample into a dry cuvette (clear plastic thing). Record the Percent
Transmittance on the data table above.
Convert percent transmittance into a decimal (example: 90% = 0.9). Record on data
table above.
Find the reciprocal of the decimal form and record that as 1/T on the data table above.
What are you going to Graph?
Make a graph of 1/T vs. molar concentration
Make a linear fit (If done correctly, this should be a straight line graph)