Experiment 2 26 January 2017 Beer’s Law

Colored solutions absorb in the visible region of the electromagnetic spectrum. Blue food coloring Wavelength, l 400 nm . . . . . . . . . . . . . . . . . 700 nm High energy . . . . . . . . . Low energy Information for Introduction!

The color wheel: relationship between appearance and absorbance Information for Introduction!

The color wheel: relationship between appearance and absorbance

Wavelength of maximum absorbance, lmax lmax is a wavelength Information for Introduction! lmax~628 nm

Beer’s Law Pertains to absorbances at lmax Absorbance is proportional to concentration at lmax A = k [Blue] where [Blue] is the concentration of Blue in molarity, mol/L solution. A = k[Blue] is like: y = mx + b … where b is zero k is actually two constants, combined, e and l e is molar absorptivity – units M-1cm-1 l is cell dimension along light path (for us, 1.00 cm) Information for Introduction!

Typical Beer’s law chart

Typical Beer’s law chart y = mx + b A = k[X] = 7.7433 x [X] Trendline and R2 value

Making a dilute a Solution McVc = MdVd 7.5 x 10-5 x Vc = Md x 50.00 mL If your assigned solution is 8.00 x 10-6 7.5 x 10-5 x Vc = 8.00 x 10-6 x 50.00 mL Vc = 5.3 mL Information for Introduction!

Using the Mohr pipet! To transfer 2.72 mL (for example), start at 0.00 mL (left), and deliver into the 50.00 mL volumetric flast the 2.72 mL. The rest (right) goes back into the original beaker, etc.

1. Draw stock solution past 0.00 mL Using the Mohr pipet! 1. Draw stock solution past 0.00 mL

Using the Mohr pipet! 2. Return solution to Stock Solution beaker until reading is 0.00 mL

Using the Mohr pipet! 3. Deliver assigned volume to volumetric flask.

Using the Mohr pipet! 4. Return excess solution to beaker of Stock Solution

Details for today Write an introduction You do not have to copy the procedure – merely refer to it. It is necessary to completely describe any modifications and changes. Each pair of students performs Part A and B and attaches spectra as part of your lab report today. This is different from lab manual. Do Part C anytime. TA will assist. Make sure you can correctly use Mohr pipet before you do Part D. Part D – you and partner will contribute one point to the class Beer’s law plot. Complete Mohr pipet activity before you submit on-line data. Submit on-line data before you leave today. You will use class data to produce a Beer’s law plot in Excel. Class data will be available at the Chm 206 website at 4 pm today. Lab reports due tomorrow at the start of class.

Entering exponentials into Google forms and Excel: No spaces. There is only this one way.

Using a cuvette Rinse with the solution you will be filling it with Fill ~3/4 full Make sure the light goes through the clear, not the “frosted” sides (line up the arrows) Make sure there are no bubbles Wipe it clean At end of lab, rinse and leave at your table

Your Lab Report TOP: Cover sheet with the TA’s initials on it MIDDLE: Lab notebook pages (carbons) – remove the left left strip of spiral holes if necessary LAST: Attached graphs (spectrum of three food colorings and Beer’s law chart) Stapled (I will have a stapler available prior to start of class) Late after the start of class (even if it is your lab partner’s fault). You can download this presentation at the Chm 206 website.