1. Let’s Talk About Beer’s Law
Beer’s Law and Concentrations

2. Solution Concentration Reminders
Concentration expresses how the amount of solute and the amount of solution compare
Our unit of choice: Molarity = mols/L
What do you notice about the solutions?

3. Why Does Color “Fade” as Concentration is Lessened?
Conc. is moles (number of ions/atoms/molecule) per space
More molecules means more light is “caught”

4. Beer’s Law
Beer’s Law quantifies the relationship between color and concentration
Beer’s Law states that the absorbance of light by a solution is directly proportional to
Molar absorptivity
Path length
Concentration

5. So, What Does This Means
It means that a graph of absorbance of light by a “kind” of solution TO the concentration is linear.
It means that predictions are accurate and reliable
We can find concentrations by comparison

6. How Do We Do This? We make several solutions with known concentrations
We determine which wavelength or color of light the solution responds to best
We measure that transmittance and/or absorbance using Spectrophotometry (technique used to measure the absorbance of light of different concentration solutions) for the known solutions and the unknown solution
We graph the knowns and interpolate for the unknown

7. A Successive Dilution Example
Design a process of successive dilution to make mls of the following solutions from a 0.85 M stock solution: M, 0.30 M, 0.10 M.

8. Why Not Transmittance?
Imagine an area enclosed by a curtain containing a mystery number of people
Each person can catch and hold 2 tennis balls
We throw 1000 balls into the area
We measure what comes out (Trans)
But we find the number of people by what doesn’t come out (Abs)
# People is related to number of balls caught (Abs)

9. Spectroscopy: The kind you can see
Spectroscopy involves the study of light that is absorbed or emitted by a substance
Visible spec. involves light we can see

10. The plan:
We’ll shine light that is absorbed well into solutions of known concentration
We’ll make a plot of known concentrations versus absorbance
We’ll test our unknown solution
We’ll interpolate to find the concentration that matches its absorbance

11. Great! How do we do that? Calibrate the machine
0 absorbance for 0 M concentration
Create a blank solution that contains all the same substances except for the substance you want to determine the concentration for
Find the best wavelength (lmax)
Test all solutions at that wavelength
Graph or use “factor”

12. Calibration of Spec 20 Allow the machine to warm up
Set wavelength to desired value (400 nm)
With sample chamber empty set %T to zero using left knob
With water (or some other solvent) in sample chamber set %T to 100% using right knob
Repeat for each wavelength

13. Testing the solutions & unknown
Once you’ve found the best wavelength (light is absorbed best) test all solutions and unknown at this wavelength
Use data to find concentration of unknown

14. Finding the unknown Suppose this graph is generated
The unknown absorbance is 0.500
Find absorbance
Across and down
The concentration is (or so)