Units The Conversion Factor

A conversion factor simply uses your knowledge of the relationships between units to convert from one unit to another. For example, if you know that there are 2.54 centimeters in every inch (or 2.2 pounds in every kilogram or 101.3 kilopascals in every atmosphere), then converting between those units becomes simple algebra. Peruse Table 2-3 for some useful conversion factors. And remember If you know the relationship between any two units, you can build your own conversion factor to move between those units. 

As with many things in life, chemistry isn t always as easy as it seems. Chemistry teachers are sneaky They often give you quantities in non-Sl units and expect you to use one or more conversion factors to change them to SI units — all this before you even attempt the hard part of the problem We re at least mcirginally less sneaky than your typical chemistry teacher, but we hope to prepare you for such deception, so expect to use conversion factors throughout the rest of this book  

The following example shows how to use a basic conversion factor to fix non-Sl units. 

Geekmajor absentmindedly measures the mass of a sample to be 0.75 lb and records his measurement in his lab notebook. His astute lab assistant, who wants to save the doctor some embarrassment, knows that there are 2.2 lb in every kilogram. The assistant quickly converts the doctor s measurement to SI units. What does she get  

Recall another algebra rule You can multiply any quantity by 1, and you ll always get back the original quantity. Now look closely at the conversion factors in the example 2.2 lb and 1 kg are exactly the same thing Multiplying by 2.2 lb/1 kg or by 1 kg/2.2 lb is really no different from multiplying by 1. 

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Note that with U.S. Customary units, the conversion factor may he required to make equations in this section dimensionally consistent g = 32.17 (lhm-ft)/lhf-s ). 

Thus, after canceling common units, the conversion factor relating slugs to lbm is 32.174 lbm/slug. 

Step 2. Insert the required conversion factors to change between units. The conversion factors will be in fraction form, and they will be orientated in such a way that we can cross-cancel the units that we don t want, and keep the units that we do want. In this particular example, we will only need one conversion factor, 1 hour = 60 minutes. We will put the unit for hours on the top, so that we will end with hours on the top. We will put the unit for minutes on the bottom, because we want to cross-cancel the minutes that we already have on the top. 

Step 1 To convert from one unit to another, use the equivalence statement that relates the two units. The conversion factor needed is a ratio of the two parts of the equivalence statement. 

We point out here the principal units, the conversion factors and the constants, which we use [ROU 99],... 

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