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 Stoichiometry: Calculations with... Introduction

IN CHAPTER 2, we saw that we can represent substances by their chemical formulas. Although chemical formulas are invariably shorter than chemical names, they are not merely abbreviations. Encoded in each chemical formula is important quantitative information about the substance that it represents.

In this chapter we examine several important uses of chemical formulas, as outlined in the "What's Ahead" box. The area of study that we will be examining is 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 use aspects of stoichiometry.

Stoichiometry is built on an understanding of atomic masses (Section 2.4) and on a fundamental principle, the law of conservation of mass: The total mass of all substances present after a chemical reaction is the same as the total mass before the reaction. The French nobleman and scientist Antoine Lavoisier (Figure 3.1) discovered this important chemical law in the late 1700s. In a chemistry text published in 1789, Lavoisier stated the law in this eloquent way: "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 the law of conservation of mass: 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 rearrange the atoms. We begin our discussions in this chapter by examining how chemical formulas and chemical equations are used to represent the rearrangements of atoms that occur in chemical reactions.

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