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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:

  • 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.

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(NO3)2∙3H2O 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:   
    .1-g     .01-g           .0001-g
  • Volume: Circle the measurement device for the water you were assigned:
    50-mL beaker        10-mL graduated cylinder          50-mL graduated cylinder
  • 50-mL 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.

Measurement
Number of Significant Figures
Mass of Copper II Nitrate g
Volume of Water mL

Class Data:

Complete this table as a class.

Group
Balance
Measurement Device
Absorbance
Percent Error
1
.1g
50mL Beaker
2
.1g
10mL Graduated Cylinder
3
.1g
50mL Graduated Cylinder
4
.01g
50mL Beaker
5
.01g
10mL Graduated Cylinder
6
.01g
50mL Graduated Cylinder
7
.0001g
50mL Beaker
8
.0001g
10mL Graduated Cylinder
9
.0001g
50mL Graduated Cylinder
10
.1g
50mL Beaker
11
.1g
10mL Graduated Cylinder
12
.1g
50mL Graduated Cylinder
13
.01g
50mL Beaker
14
.01g
10mL Graduated Cylinder
15
.01g
50mL Graduated Cylinder
16
.0001g
50mL Beaker
17
.0001g
10mL Graduated Cylinder
18
.0001g
50mL Graduated Cylinder
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.

  1. 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.
  2. 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.
  3. 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.