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LAB in Concentration, Molarity, Measurements, Significant Figures, Beer's Law. Last updated June 11, 2021.
Summary
In this lab investigation, students will create a copper(II) nitrate solution. Each group will be given a different measurement device in order to see how the accuracy of the preparation of the solution is affected by the limitations of the measurement device. The goal is for students to have a true understanding of why significant figures are important.
Grade Level
High School
NGSS Alignment
This lab will help prepare your students to meet the performance expectations in the following standards:
 HSETS12: Design a solution to a complex realworld problem by breaking it down into small, more manageable problems that can be solved through engineering.
 Scientific and Engineering Practices:
 Analyzing and Interpreting Data
 Planning and Carrying Out Investigations
Objectives
By the end of this lab, students should be able to
 Recognize the importance of significant figures.
 See that there are limitations of some measurement devices.
 Better understand the concept of concentration and molarity.
 Know that there is a relationship between the absorbance and concentration of a solution.
Chemistry Topics
This lab supports students’ understanding of
 Significant Figures
 Measurement
 Solutions
 Molarity
 Concentration
 Beer’s Law
Time
Teacher Preparation: 20 minutes
Lesson: 90 – 120 minutes
Materials
 Copper(II) Nitrate trihydrate (Cu(NO_{3})_{2} ∙ 3H_{2}O)
 Colorimeter or Color Spectrophotometer
 Multiple balances (suggestion to have one or more that can measure to 0.1g, 0.01g, and 0.0001g)
 Cuvette (one per group)
 Beakers (50mL)
 Graduated cylinders (10mL and 50mL)
 Note: Use the materials that you have available. If you only have one type of balance, then use four or five different sizes of glassware.
Safety
 Always wear safety goggles when handling chemicals in the lab.
 Students should wash their hands thoroughly before leaving the lab.
 When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
 Review the MSDS for Copper(II) Nitrate trihydrate. Keep away from heat, sparks, hot surfaces and open flames.
Teacher Notes
 Teachers should create a 0.200M Copper(II) Nitrate solution as a control sample to serve as the “theoretical value”. I would recommend making this solution with a more accurate device than the students can access. For example, use a pipet or buret to measure your water for increased accuracy. Collect the absorbance value of this solution for use in percent error calculations.
 When calculating percent error the students should use the absorbance of the control sample created by the teacher as the “theoretical value” and the absorbance of the sample they made as the “experimental value.”
 This lab can be flexible in terms of materials available. For example, if you only have one type of balance, you may choose to use four or five different pieces of glassware for measurements. Be sure to update the data table on the student handout to reflect the measurement devices made available to each group.
 Teachers may choose to have fewer scenarios, so more groups are completing the same measurements. Again, make sure to update the data table on the student handout to reflect the materials made available to each group.
 I suggest and like to have the students work in pairs for this lab, but you may want them to work as individuals so you get more results.
 The data table is currently designed for 18 groups of students, with 9 different measurement combinations. This will allow two results to be collected for each combination of tools used.
 Depending on the level of your students and/or the length of time you spend on discussions this lab may take longer than 90 minutes.
 This lab isn’t designed with an emphasis on Beer’s Law, or the student use of a colorimeter. The teacher can complete this part of the data collection, and/or show each student group how this is done. Additionally, should explain the relationship between absorbance and concentration (this information is not outlined in the lab materials for students).
 After the students have finished sharing data and completing the table:
 Students should answer the three questions.
 Have small discussion circles (combine 23 groups) where the ideas are studentdriven. I always try to be careful about giving away the answers as the goal is to have the students develop the concepts. The goal is for the students to understand that the more precise the measuring device the more accurate results they will get. Students should also understand when we use more precise measuring devices our results will have more significant figures.
 I give the students time to revise their answers to the questions based on their discussion.
 Lastly, I ask the students to complete the conclusion section (this would be a great homework assignment to discuss at the start of the next day or possibly a part that I would grade to see if the students have really mastered the concept).
 Students should answer the three questions.
 The class discussions you have can be very important. If a group is using less accurate measuring devices obtains great results, you may have a discussion about whether those results can be obtained consistently.
 If your students have not performed many measurements, you may want to discuss the accuracy of measurement markings on a device, as well as estimating one digit.
 A possible extension idea would be to have students investigate the effect of insecticides on the environment.
For the Student
Lesson
Background
You work for an insecticide company and your company has been cited by a local municipality because the molarity of the Copper(II) Nitrate produced for the insecticide is too concentrated. Your boss is concerned that the variety of devices used to make the Copper(II) Nitrate solution are not accurate enough to successfully meet the environmental standards each time.
In order to fully investigate this concern, she would like each group to make a sample of 7.50mL of 0.200M Copper(II) Nitrate solution. Each group will be assigned to use specific measuring devices to make their sample. Once the solutions have been created, she will be testing the absorbance of the solution in order to see if the solution has an acceptable molarity value.
Prelab Calculation
Calculate the mass of Copper(II) Nitrate you will need to make 7.50mL of 0.200M solution. Note that Copper(II) Nitrate is a trihydrate. Use the formula, Cu(NO_{3})_{2}∙3H_{2}O in the calculation. When you have finished your calculation, have your teacher check your results.
Materials
 Mass: Circle the type of scale your group was assigned:
.1g .01g .0001g  Volume: Circle the measurement device for the water you were assigned:
50mL beaker 10mL graduated cylinder 50mL graduated cylinder  50mL beaker (only used for mixing solution)
 Stirring Rod
 Copper(II) Nitrate
 Distilled Water
Safety
 Always wear safety goggles when handling chemicals in the lab.
 Wash your hands thoroughly before leaving the lab.
 Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
 Keep Copper(II) Nitrate away from heat, sparks, hot surfaces and open flames.
Procedure
Write the procedure that your group will follow to make a solution with proper molarity. Be sure to include the glassware/equipment you will be using in each step; you need to be specific (example: “Use the 50mL beaker…” instead of “Use the beaker…”). Make sure you include every step you take. Your final step should be, “Pour the Copper(II) Nitrate solution from the 50mL beaker into the cuvette.” Once you have finished your procedure get permission from your teacher to start the investigation.
Group Data:
Complete this table with your group.



Mass of Copper II Nitrate  g  
Volume of Water  mL 
Class Data:
Complete this table as a class.













































































Control sample  NA 
Error
Calculate the percent error for your sample. Once you complete your calculation be prepared to share your results with the class.
Analysis
After your class has finished filling out the table, answer the following questions.
 Find a general correlation between the sensitivity of the device used and the percent error for groups that seemed to have the most error. Then write a statement describing your understanding of this correlation and include a specific example or two that supports this statement.
 Write a statement describing the correlation between sensitivity of the device and percent error for groups that seemed to have the smallest percent errors. Cite data that support this correlation.
 How are significant figures applicable to this lab? Be sure to explain why significant figures are important to the measurements we make and the calculations to which we apply those measurements.
Conclusion
Your boss has suggested she may be purchasing new equipment for producing the Copper (II) Nitrate solution. Following this investigation what recommendation would you make to your boss to help prevent future violations? She has asked you to keep in mind that the more accurate the device the more expensive it will be.