1. Beer’s Law Colorimetry Colligative Properties Review

2. Objectives
Today I will be able to:
Apply Beer’s Law to determining the concentration of an unknown solution
Informal assessment – monitor student questions and interactions as they complete the practice problems
Formal assessment – analyze student responses to the practice and exit ticket

3. Lesson Sequence Evaluate: Warm – Up
Explain: Beer’s Law Notes/Sample Data
Elaborate: Colligative Properties Review
Evaulate: Exit Ticket

4. Warm Up Think back to the Kool-Aid lab…
What happened to the color of the solutions as the molarity increased?
How would this observation impact the amount of light transmitted

5. Objectives Today I will be able to:
Apply Beer’s Law to determining the concentration of an unknown solution

6. Homework Wear closed toe shoes Wednesday
Study for Solutions Exam – Friday, May 15

7. Agenda Warm – Up Beer’s Law Notes/Sample Data
Colligative Properties Review
Exit Ticket

8. Beer’s Law Notes, Intro to Colorimetry

9. Beer’s Law
Beer’s Law quantifies the relationship between color and concentration
Formula:
A = ebc
e is molar absorptivity (unique for a given compound)
b is path length
c concentration

10. We can use Beer’s Law to Determine the concentrations of unknown solutions

11. How do we do this?
We use a technique called spectroscopy

12. Spectrophotometer

13. How Do We Do This in lab?
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 for the known solutions and the unknown solution
We graph the knowns and interpolate for the unknown

14. Sample SCenario

15. Sample Scenario
A series of standard solutions containing a red dye was made by diluting a stock solution and then measuring the percent transmittance of each solution at 505 nm (greenish blue). This wavelength was selected by examining its absorption spectrum. If the solution looks red, it is absorbing red's complementary color of light, which is greenish blue. The results, after conversion to absorbance, are shown below.

16. Step 1 - Prepare Dilutions and calculate absorbance
Solution
Absorbance
Blank
0.00
Standard # 1
0.24
Standard # 2
0.50
Standard # 3
0.72
Standard # 4
0.99
Unknown Sample
0.39

17. Step 2 – Calculate Concentrations using Beer’s Law A = ebc
Solution
Absorbance
Concentration
Blank
0.00
0.00 M
Standard # 1
0.24
0.15 M
Standard # 2
0.50
0.30 M
Standard # 3
0.72
0.45 M
Standard # 4
0.99
0.60 M
Unknown Sample
0.39
???? M
Note: You would need to know the e-value and calculate the pathlength (b)

18. Step 3 – Create a Concentration vs Absorbance Standard Curve and find line of best fit

19. Step 4 – Determine the concentration of the unknown using the line of best fit

20. Finish Colligative Properties Calculations from Friday

21. Exit Ticket
What is the purpose of creating a standard curve when using a spectrophotometer?
What steps do you take to determine the concentration of an unknown?