Metric-English conversions

Beginning students often regard the metric system as difficult because it is new to them and because they think they must learn all the English-metric conversion factors (Table 2-3). Engineers do have to work in both systems in the United States, but scientists generally do not work in the English system at all. Once you familiarize yourself with the metric system, it is much easier to work with than the English system is. 

Instructors often require English-metric conversions for two purposes to familiarize the student with the relative sizes of the metric units in terms of the more familiar English units, and for practice in conversions (see Sec. 2.4). Once you really get into the general chemistry course, the number of English-metric conversions that you do is very small. 

English-metric conversions are most often presented, if at all, to give an idea of the size of the metric unit. They are not used very much in this course. 

English units are still common in some countries, while people in other countries (and the scientific community everywhere), use metric units almost exclusively. Unit analysis provides a convenient method for converting between English and metric units. Several of the most commonly needed English-metric conversion factors are listed in Table 8.1 on the next page. Because the English inch is defined as 2.54 cm, the number 2.54 in this value is exact. The numbers in the other conversion factors in Table 8.1 are not exact. 

Memorizing other English-metric conversion factors will save you time and effort. For example, if you know that 1.609 km = 1 mi, the problem becomes much easier. 

Write the English-metric conversion factors listed on Table 8.1. 

English-Metric Conversions—More Conversion Factors... 

Perform the requested English-metric conversions, expressing the answers in scientific notation to the allowed number of significant figures. 

If you answered incorrectly, review English-Metric Conversions, page 24. 

Note The number 39.37 has only four sig figs in spite of the fact that it is a conversion factor (See Rule 5 in Table 2.2). English-metric conversion factors, in generai, are not exact numbers, and thus must not limit the number of sig figs in the answer to the calculation in which they are used. In this case, the answer should have four sig figs. It may be possible to find a more precise number in a handbook in order to allow the measured number, rather than the conversion factor, to limit the sig figs in the answer. One exception to this general rule is the equality 2.54 cm = 1 in. This is an exact number because it defines the inch. 

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