Humans have long been interested in how much of something they have. Ancient commerce was often conducted by trading goods for a certain weight of gold or silver. Both religious and civil statutes forbade the use of false weights in conducting business. In chemistry, we routinely weigh materials to determine how much of a chemical we are adding to a reaction. The amount of product is often weighed to determine reaction yield. Chemical solutions are prepared by weighing a specified amount of material and dissolving it in a defined volume of solvent. Weight and volume are two different ways to measure how much of a substance is present. In this chapter, we will explore how to express amounts of materials in a way that indicates the number of atoms or molecules contained in a given sample.
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