1. Sample AP Model Drawing Question

3. Today:
How is the color of a solution related to concentration and how can that be used to our advantage?

4. I0 I1 Concentration and Color: Solution 1: Low concentration
Incident Light I0 I1
(The wavelength must be specific to this solute)

5. I0 I2 Concentration and Color:
Solution 2: Higher concentration, same solute
Incident Light I0 I2

6. I1 > I2 Concentration and Color:
Solution 1 = lower M, Solution 2 = higher M
How does the observed light compare between the two solutions?
Solution 1
Solution 2
I1 > I2

7. Concentration and Color:
Why do we care?
The amount of light that is able to pass through a solution can be used as a way to measure the concentration!

8. Concentration and Color:
Some terms …
Transmittance:
A measure of the light that was able to pass through the sol’n
Absorbance:
A measure of the amount of light that was absorbed by the solution
A sol’n with high transmittance, will have low absorbance.

9. Concentration and Color: Beer-Lambert Law A = a x b x c
where A is the absorbance
a = constant, called molar absorptivity
b = path length, in cm
c = concentration, in mol/L
This law shows that there is a linear relationship between absorbance and concentration

10. Concentration and Color:
Beer-Lambert Law

11. Example Problem: What color light is a red dye likely to absorb?
The following absorbances were measured for different concentrations of red dye at 505 nm (greenish-blue)
Solution
Concentration
Absorbance
blank
0.00 M
0.00
standard #1
0.15 M
0.24
standard #2
0.30 M
0.50
standard #3
0.45 M
0.72
standard #4
0.60 M
0.99
Unknown
???? M
0.39
Determine the M of the unknown sample.

12. Example Problem: Graph the known concentrations …
Solution
Concentration
Absorbance
blank
0.00 M
0.00
standard #1
0.15 M
0.24
standard #2
0.30 M
0.50
standard #3
0.45 M
0.72
standard #4
0.60 M
0.99
Unknown
???? M
0.39
Example Problem:
Graph the known concentrations …
Determine the M of the unknown sample.

13. Try #1 and #2 on your own: