Unidades de medida

Units of Measurement
Many properties of matter are quantitative; that is, they are associated with numbers. When a number represents a measured quantity, the units of that quantity must always bespecified. To say that the length of a pencil is 17.5 is meaningless. To say that it is 17.5 centimeters (cm) properly specifies the length. The units used for scientific measurements are those of themetric system.
The metric system, which was first developed in France during the late eighteenth century, is used as the system of measurement in most countries throughout the world. The United Stateshas traditionally used the English system, although use of the metric system has become more common in recent years. For example, the contents of most canned goods and soft drinks in grocery storesare now given in metric as well as in English units as shown in Figure 1.16.
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Figure 1.16 Metric measurements are becoming increasingly common in the United States, as exemplified by the volumeprinted on this container.
SI Units
In 1960 an international agreement was reached specifying a particular choice of metric units for use in scientific measurements. These preferred units are calledSI units, after the French Système International d'Unités. The SI system has seven base units from which all other units are derived. Table 1.4 lists these base units and their symbols. In thischapter we will consider the base units for length, mass, and temperature.
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Prefixes are used to indicate decimal fractions or multiples of various units. For example, the prefix milli- representsa [pic]fraction of a unit: A milligram (mg) is [pic]gram (g), a millimeter (mm) is [pic]meter (m), and so forth. Table 1.5 presents the prefixes commonly encountered in chemistry. In using the SIsystem and in working problems throughout this text, you must be comfortable using exponential notation. If you are unfamiliar with exponential notation or want to review it, refer to table below....