Module 5: Chemical Quantities and Composition

Many concepts in chemistry deal with the behavior of chemicals – things like color, appearance, and reactivity.  However, in the study of chemistry, quantity and composition are equally as important. For example, it may be easier (or even only possible) to predict how something will behave, if you know how much of it you have.

In this module, you will explore the concept of the mole, which is a basic unit that represents a large number of atoms. In addition, you will learn how to calculate the gram formula mass of a compound. Next, you will study percent composition, which helps you determine the individual percent of mass of each element in a compound.  Finally, you will examine empirical and molecular formulas and how to calculate them yourself.

Getting Started

Often, chemists have to calculate the number of tiny atoms in a substance and then use this number in different types of calculations. How could you count something without being able to touch each individual item? What if you are unable to see each individual item?

Examine the jar of candy in the image on the right. How many individual pieces of candy are in the jar? Take a few moments to devise a way to quickly determine the number of candy items in this jar without counting each individual piece.

Key Vocabulary

To view the definitions for these key vocabulary terms, visit the course glossary.

atom	molar mass (M)
Avogadro’s number	molecular formula
empirical formula	particle
gram formula mass	percent composition