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Introduction

We have discussed atoms in the previous chapter, so let's now turn our attention to what happens to matter when it undergoes chemical changes. In the late 1700s a French nobleman named Antoine Lavoisier observed that the total mass of all substances present after a chemical reaction is the same as the total mass before the reaction. This observation, known as the law of conservation of mass, is one of the fundamental laws of chemical change. In 1789 Lavoisier published a textbook on chemistry in which he stated: "We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment." With the advent of the atomic theory, chemists came to understand the basis for this law: Atoms are neither created nor destroyed during any chemical reaction. Thus, the same collection of atoms is present both before and after a reaction. The changes that occur during any reaction merely involve rearrangement of the atoms. The law of conservation of mass will serve as one of the guiding principles in our discussions in this chapter.

The chapter begins with an examination of chemical equations and how they are balanced. We will then consider some simple kinds of reactions, including combustion reactions. To gain some understanding of the quantitative significance of chemical formulas and chemical equations, we next consider atomic and formula weights and the mole concept. Finally, we will consider how the mole concept can be used to determine chemical formulas and to determine the relative quantities of reactants and products involved in chemical reactions.

The quantitative nature of chemical formulas and chemical reactions involves an area of study known as stoichiometry (pronounced stoy-key-OM-uh-tree), a name derived from the Greek stoicheion ("element") and metron ("measure"). Stoichiometry is an essential tool in chemistry. Such diverse problems as measuring the concentration of ozone in the atmosphere, determining the potential yield of gold from an ore, and assessing different processes for converting coal into gaseous fuels all involve aspects of stoichiometry. What we learn in this chapter forms a foundation of much of the chemistry that we encounter in later chapters and in the laboratory portion of the course.






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