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ACTIVITY in Molecular Formula, Atomic Mass. Last updated June 05, 2017.

Summary

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

Grade Level

High school

Objectives

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

  • determine a molecule’s molecular formula from its formula mass.

Chemistry Topics

This lesson supports students’ understanding of

  • Atomic mass
  • Molecular formula

Time

Teacher Preparation: 10 minutes
Lesson: 20 minutes

Materials

  • Playing cards, sorted for random pair assignment.
  • 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.

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.
  • Students should know before this activity how to determine formula masses, given chemical formulas and the atomic masses on the Periodic Table.
  • The competitive style of this activity lends itself to numerous modifications, like the posting of tournament brackets on the board, lifelines, and additional tasks.

For the Student

Lesson


Purpose

To master the concept of a formula mass by requiring students to work backwards from formula masses to chemical formulas in a setting that motivates through both cooperation and competition. Students will determine molecular formulas given formula masses and a limited number of atoms.

Materials

  • Playing cards, sorted for random pair assignment.
  • 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.
Procedure

Introduction

Students will be handed a random playing card as they walk into the room. The first order of business will be to make pairs of students according to the card they got; these pairs will be the competition teams.

Explain the game: it is a competition to build chemical formulas from formula masses. At this point, students should have experience determining formula mass from a chemical formula – today, they will be doing the reverse. Show them the molecule sets laid out on the lab desks in six pairs. After ensuring that they understand, assign them to their first competitions.

Lesson Development

At each of six lab tables, two teams will be racing to build three physical molecular models according to a card that lists three chemicals by name and formula mass. They will not be able to deduce the formula from the name, but they will use the periodic table to build molecules that match the formula masses on the card. Each team will be supplied with just enough atoms and bonding sticks to build the three chemicals described on the card. Each of the lab tables will have two identical sets of three chemicals. Teams may claim victory by notifying the teacher and having their three models checked.

Atoms are coded by colors, and the color key will be posted on the whiteboard.

After a competition is over, the teams will wait until a new lab table is open and another opposing team is ready. Then another competition can begin. The team with the most wins at the end of the official time (to within ten minutes of the period’s end) will be deemed the winner.

Closure

To close, volunteers for a final competition will be selected. This competition will require contestants to work alone and build a larger molecule, without the aid of a limited set of atoms. The teacher may also participate.

Interdisciplinary Connections

The skills developed here translate into biochemistry, where unknown proteins must be identified by their molecular weight and other factors.

Connections to Standards

Reference to PA Standards

3.1.10.B: Describe concepts of models as a way to predict and understand sci/tech.

3.2.10.D: Identify and apply the technological design process to solve problems.

3.4.10.A: Explain concepts about the structure and properties of matter.