AP Chemistry Phet Beer's Law Activity

AP Chemistry
Beer's Law Activity

By David Burdrick

Directions

Directions

·        Follow the link provided in a Canvas page directions to the Phet activity in a new tab

·        Then work through the activity and complete each step and question.

Spectrophotometers are used by chemists to study how much light at a specific wavelength is absorbed as it passes through a colored solution.

Open the Beer's law simulator using the link provided in the canvas activity in a separate window.

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Choose the right wavelength

1.       Reset the simulation

2.       Choose nickel (III) chloride as the solute

3.       Turn on the light

4.       Set the meter to absorbance (type of measurement used in Beer’s Law)

Fill in the Blank

What is the preset wavelength ?

• Go to the Simulator

• To see why this wavelength was chosen, change the wavelength setting to variable.

Multiple Choice

Now move the slider left and right of the preset wavelength and see what happens to the absorbance. The preset wavelength corresponds to the _______________________ absorbance. This is done to minimize percent error.

• Minimum

• Maximum

• Has no Significance

Multiple Choice

The best wavelength to use in a Beer’s law lab is usually a color that is complementary to the color of the solution. What do you think the light color will be for a solution with a yellow color?

• Red

• Orange

• Yellow

• Green

• Purple

• Go to the simulator

• change the solution to potassium chromate.

• Click on the variable wavelength.

Multiple Choice

What is the default wavelength listed?

• 643 nm (Red)

• 411 nm (Yellow)

• 411 nm (Purple)

• 643 nm (Purple)

Path Length

a. Reset the simulation. Turn on the light and set the meter to absorbance.
b. Move the ruler to measure the cuvette width

Fill in the Blank

Measure (to proper precision) the cuvette width? ________ cm   This is the measured path length (b) in the Beer’s Law equation. Do not include units in your answer and remember the measurement rule - you can estimate to 1 digit better than the smallest division!

.

• Go to the simulator

• Turn on the light

• Grab and drag the right side of the cuvette making it larger

Multiple Choice

What happens to the absorbance?

• increases

• decreases

• stays the same

Multiple Choice

Why? Because the light has to travel thru more solution, it will collide with  ________________ solute particles causing a _______________  absorbance.

• Less/Higher

• Less/Lower

• More/Higher

• More/Lower

• Reset the simulator

• Turn on the light and set the meter to absorbance

• Set the wavelength to varied and adjust to 500 nm

​Other Variables

Poll

At the same concentration (100 mM which is 0.1 M), should the measured absorbance at 500 nm for three red solutions (drink mix), cobalt (II) nitrate and cobalt (II) chloride) be similar.

• Yes

• No

Fill in the Blank

What is the measured absorbance at 500 nm for drink mix?

.

Fill in the Blank

What is the measured absorbance at 500 nm for cobalt (II) nitrate?

.

Fill in the Blank

What is the measured absorbance at 500 nm for cobalt (II) chloride)?

.

Multiple Choice

These absorbances vary because while each of the solutions have the same concentration, path length, and wavelength of monochromatic light, they will all have different _________________________________ (which is a constant for that depends upon the identity of the solution in Beer's Law calculations).

• Solution colors

• absorptivity coefficients

• refractive indexes

• solution density

​Concentration

Reset the simulator and turn on the light and set the meter to absorbance

Fill in the Blank

13.  For 0.10 M (100 nM) drink mix at the preset wavelength, what is the absorbance? __________ nm

.

Multiple Choice

If the concentration is increased, what should happen to the absorbance? ____________________

• It will increase.

• It will decrease.

• It might increase or decrease.

Multiple Choice

Why? Because the light has to travel thru a more concentrated solution, this will cause the photons of light to collide with more particles causing _______________ photons to be absorbed which causes a _______________ absorbance

• less, lower

• less, higher

• more, lower

• more, higher

Multiple Choice

What is the absorbance of drink mix if the concentration is 0.15 M (150 nm)? __________

• 0.51

• 0.49

• 0.65

• 0.76

Fill in the Blank

What is the absorbance of drink mix if the concentration is 0.40 M? __________

.

​Take a screen shot or copy the following data on a plank sheet of paper

Draw

Plot the data from the previous slide on the graph

Multiple Choice

When the same solution and cuvette are used, the absorptivity coefficient and path length will be _________________ (constant or variable).

• Constant

• Variable

Multiple Choice

This leaves absorption and concentration as the only remaining variables in Beer’s Law. A graph of these two variables will be __________________. (see the graph you generated)

• Curved

• Linear

Open google sheets, excel, or any graphing program.  Using the data above graph the absorbance versus concentration. Is your graph linear? __________ here

Multiple Choice

What is the slope of the best fit line (to two significant figures)? __________ (units will be 1/M but do not include them in your reported answer)

• 1.0

• 3.5

• 4.7

• 5.2

Fill in the Blank

If the answer for the slope in in the previous question was 5.2 (it's not!),  calculate the absorptivity coefficient (Ɛ) of cobalt (II) nitrate. The unit will be 1/cmM but do not include units in your answer). (Hint: the math should be very easy)

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​Another Way is .......

Fill in the Blank

Use this method to calculate the molar concentration of "copper (II) sulfate" if it has a measured absorbance of 2.17. The absorbance of a 0.10 M solution is 0.96. Round your answer to correct significant figures and do not include units.

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​End of Activity
Make sure your score downloads to canvas before closing the wayground program