Summary

In this activity, students determine molecular formulas given formula masses and a limited number of atoms.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
    

Objectives

By the end of this lesson, students should be able to

• determine a molecule’s molecular formula from its formula mass in a setting that motivates through both cooperation and competition.

Chemistry Topics

This lesson supports students’ understanding of

• Atomic mass
• Molar mass
• Molecular formula
• Naming compounds

Time

Teacher Preparation: 15 minutes
Lesson: 30 minutes

Materials

• Molecular model building kits, sorted into six sets which each contain two identical subsets.
• Building cards with chemical names and formula masses for each subset.
• Periodic tables (or a list of atomic masses for the atoms being used)

Safety

There are no special safety considerations for this activity.

Teacher Notes

• Prior to this activity, students should be made familiar with molecular model building kits and the formula mass concept.
• Prior to this activity, it may be helpful to have students analyze pre-built molecular models and determine their chemical formulas and formula masses. During this activity, they will be doing the reverse: determining a chemical formula from formula mass. You could also do Set 1 as a class as practice.
• Students should know before this activity how to determine formula masses, given chemical formulas and the atomic masses on the Periodic Table. All atomic masses used for the answer key were rounded to two decimal places.
• The competitive style of this activity lends itself to numerous modifications, like the posting of tournament brackets on the board, lifelines, and additional tasks.
• Have two of each set of atoms and the corresponding lists of molecules set up at different lab station/table and have students rotate around the room once they’ve finished with a set of molecules.
• Post the color key for atoms on your board or provide it to students.
• If you wish the students to actually build the molecules, they would need to know about double/triple bonds for some of the compounds, and some molecules might have more than one correct structure. It might be simpler, if you only want to have students find the masses, to just have them choose the atoms that will add up to that mass (not necessarily connect them with the model kit bonds).
• The analysis questions will help students recognize patterns in naming conventions for organic compounds. They can be omitted if you are short on time or don’t want to get into naming organic compounds.
• In this activity, students work in teams of two against another team of two students to race to finish building their molecules first. If you have more/fewer students, you can add/remove students. You can pre-select student groups or assign them randomly.