# Writing Conversion Factors

This worksheet is intended to be used as a "Guided Instructional Activity" (GIA). Students read a statement that gives a either a conversion factor or a pair of related measures and then write the information as two equivalent fractions ("conversion factors") and as an equality. In each representation, students are directed to give the numeral of the measure, unit, and identity of the chemical. It is designed to help develop good habits in representing molar mass and other conversion factors, and to emphasize the idea that a conversion factor has a numerator and denominator that "name" identical quantities using different measures.

This activity may also be used as a homework assignment.

This worksheet is used early in the course, as usually just before the molar mass concept is introduced. It is an essential component of lessons leading to stoichiometry. A complete stoichiometry unit is described in Stoichiometry is Easy.

Concepts:
dimensional analysis/factor label method
molar mass
stoichiometry
unit conversion
Concepts:

Unit (dimensional) analysis. Writing conversion factors. Writing measures with numeral, unit, identity of chemical.

Procedure time:
50 minutes
Prep time:
10 minutes
Time required:

35 to 45 minutes.

Materials:

Make a copy of the GIA for each pair of students. The GIA may also be projected for the entire class using an overhead or document projector, or copies may be placed inside sheet protectors and reused from year to year.

Background:

The proper use of conversion factors makes the common processes of changing mass, number of particles, or volume to moles. Students who learn "dimensional analysis," (or unit analysis) the method of clearly and correctly identifying each measure or constant they wish to use with numeral, unit, and chemical identity, are much more successful in performing these basic math problems. They quickly understand that by being specific with each measure or constant they can determine for themselves, without relying on some chart or mnemonic device, when they have a unit conversion problem set up to give the correct answer.

Procedure:

Guided Instructional Activities (GIAs) were part of the “Mastering Chemistry on the Web” (MCWeb) program, a part of the National Science Foundation Molecular Science Project (http://www.molsci.ucla.edu). Dr. Patrick Wegner (California State University, Fullerton) developed these POGIL-like (Process-Orineted Guided-Inquiry Learning) activities for use in preparatory and general chemistry classes. While some of the activities are, like POGIL real guided inquiry, many are simply cooperative learning activities that give students an opportunity to work toward a common goal while discussing and practicing skills of particular interest.

The usual procedure is to assign students randomly into groups of two (which change for each activity). They work at tables of two groups each. Students who have difficulty with an item are to consult their partner, then the other group at their table, and then may ask the teacher. Students should not move from group to group. The teacher circulates around the class making sure each group is on task, answering questions,  and revealing the answers a little at a time so students can confirm they are correctly doing what they were asked to do. This put students in charge of their own learning, gave some the opportunity to “teach” others, and allows the teacher more time to work with students who needed extra attention during class time in a non-threatening environment. At the same time, students police each other to make certain everyone (OK, nearly everyone) is on task.

Questions:

See the worksheet.

Preparation:

Make copies as indicated in the "Materials" section.

Created by Dr. Patrick Wegner, California State University, Fullerton.

## NGSS

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Summary:

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Assessment Boundary:

Assessment does not include complex chemical reactions.

Clarification:

Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem - solving techniques.