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:

• HS-ETS1-2: Design a solution to a complex real-world 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(NO₃)₂ ∙ 3H₂O)
• 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 2-3 groups) where the ideas are student-driven. 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).
• 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.