Investigation 2 – How Can Color Be Used to Determine the Mass

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Investigation 2 – How Can Color Be Used to Determine the Mass
Investigation 2 – How Can Color Be Used to Determine the
Mass Percent of Copper in Brass
Pre-Lab Guiding Questions/Simulations
AT HOME - to be turned in on DAY ONE in class which will be Oct 19th or Oct 20th
Step 1: Take notes as you watch the video on EdPuzzle!
Here are some key terms you will want to make sure you can define and use.
o transmittance,
o absorbance,
o wavelength,
o what species is absorbing the light (metals ions with characteristic
colors),
o relationship between concentration and absorbance
Step 1B: On the wiki you will also find questions to answer that go along with the video.
Please complete on a separate sheet of paper. Turn in class on Day One
DAY ONE: IN CLASS – (45min) Oct 19th or Oct 20th
Step 2:
1) Your group will be given two different salt solutions of the exact same
concentration.
2) You are going to analyze the solutions to determine the wavelength of maximum
absorbance.
3) You will need to create a data table to collect your information as well as class
data. There are six solutions so please make your data table the appropriate
size. Below you will find an example. THIS MUST BE DONE BEFORE YOU TEST!!
4) Follow the directions for using the SpectroVis to run your two trials.
Group #
Best Wavelength
Absorbance Value
Species that did
absorbing
(Data table should be included in formal lab report)
Direction for Using SpectroVis Plus Spectrophotometer
Getting Started
1) Use the USB cable to connect the spectrophotometer to the LabQuest2.
2) Turn on the LabQuest2 and it will launch the meter screen.
3) Wait five minutes for the lamp in the spectrophotometer to warm up, the
LabQuest won’t allow you to proceed w/o doing this step
To Calibrate
1) Choose Calibrate > USB: Spectrophotometer from the sensors menu.
2) Fill a cuvette ¾ full of dH2O, place the lid on the cuvette.
3) Make sure the cuvette is dry and clean. Use a Kimwipe, they are lint free.
4) Align the cuvette so a clear side of the cuvette is facing the light source.
5) Follow the instructions in the dialog box to complete the calibration and then
click OK.
To Find the Wavelength of Maximum Absorbance
1) Make sure you calibrated the machine; you only have to do this once.
2) Fill a cuvette ¾ full of solution to be tested, place the lid on the cuvette.
3) Make sure the cuvette is dry and clean. Use a Kimwipe, they are lint free.
4) Align the cuvette so a clear side of the cuvette is facing the light source.
5) Start data collection by tapping on the green START button in the lower left hand
corner of the screen.
6) Allow the data to be collected, very quick less than 10 seconds.
7) Tap the red STOP button to end data collection.
8) Select a wavelength. The machine will automatically select the wavelength of
maximum absorbance for you. You can on the graph to see all of the
wavelengths collected or go to the X/Y chart on the upper right to see a data
table of information.
9) Save your data so classmates can review it.
Step 4: Answers should be included in final lab report.
1) Look at your plots and classmate plots for the 6 different solutions
2) Can you identify the species doing the absorbing? Is it the metal ion, the
polyatomic ion or both?
3) How do you identify it?
Step 5: Answer these questions at home for your final lab report.
1) Why do we want to use a particular wavelength when determining the
absorption of a particular chemical species? Is it important to measure the
absorbance for all wavelengths from 380-950 nm?
2) Use the Beer-Lambert Law
A = abc
o A = light absorbed
o c = concentration of ions
o b = thickness or depth of cuvette (generally 1cm)
o a = molar absorptivity constant, depends on wavelength and substance
to calculate the molar absorptivity constant for the four solutions that absorbed
light.
Pick your day to work after school with your partner and sign up in class. Oct 26 and 27
will be with Gill and Oct 25th and Oct 28 will be with Long. Maximum of 14 set if
partners each day. Also make sure you have your procedure ready to go, info is below
for after school work.
After School: Investigation (45min-60min)
Refer to your lab manual page 25, steps 1-3 should be written in your lab notebook.
Please make sure to copy in the equation below and leave space to complete your
stoichiometry calculation.
First part of the lab involves a redox reaction,
8H+ + 2NO3- + 3Cu(s)  3Cu+2 + 2NO(toxic) + 4H2O
Make sure to create a small data table for the mass of copper and the amount of nitric
acid used.
Before Day Two in class
Go to pg 25 in lab manual and add #4 to your procedure. You then need to complete
the appropriate calculations for a serial dilution. In a serial dilution you take the original
solution to make the next solution. For example, you will be using the .400M to make
the .200M. Then you use the newly made .200M to make the .100M, and so forth. You
should use the M1V1=M2V2 to calculate the volume of solution needed and then add
water to total 10mL. I need to see the calculations before you mix the solutions.
DAY TWO – In Class
1) Make your 4 new standards to use with the original standard to create a plot to
determine the concentration of your unknown. Use graduated cylinders and the
large test tubes provided.
2) Test your standards using the directions below.
Testing the Standards and Unknown
To Study Absorbance vs Concentration
1) Follow the calibration steps and then the steps to find the wavelength of
maximum absorbance
2) On the meter screen, tap Mode (right side, center). Change the mode to Events
with Entry.
3) Enter the Name (Concentration) and Units (mol/L). Select OK.
4) A message will appear warning you to either save or discard the full spectrum
run. You can discard this data.
5) Place your first solution in the spectrophotometer. Start data collection by
hitting the green START button. After the absorbance reading stabilizes, tap
KEEP (to the left of the STOP button).
6) Enter the concentration of the solution and select OK.
7) Repeat steps 5-6 until all of the sample with a known concentration have been
tested.
8) After you have tested the final standard, tap the red STOP button to end data
collection.
9) Record your concentrations and absorbances on your data table.
10) Test your unknown copper sample from your gun shell casing. Record this on
your data table.
11) Pick up with step 8 on page 26 of your lab manual. Start here: Using the
absorbance and concentration for the five standard solutions, prepare a graph of the
absorbance versus the concentration… follow through to the end of the lab.
Continue through to step 10.
Clean-up
Standards can be rinsed down the drain.
Unknown needs to be neutralized with baking soda, then into waste container to
evaporate. You can get a large beaker to neutralize your remaining solution and then
dump into the class waste container
Post Lab –
Complete the Data Collection and Computation, although you have already done the
work for #1 and #2. So start with number 3 and continue on.
Argumentation and Documentation
Please enter your data into the appropriate survey, the link will be provided on the wiki
page. I will then post the class data online. Then calculate the average for each
method. Which method provided a more precise value? Justify your answer.
Post Lab Assessment
Complete the problem on page 28.

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