Consider a pin measuring 2.85 centimeters in length. What is its length in inches? To accomplish this conversion, we must use the equivalence statement 2.54 cm = 1 in If we divide both sides of this equation by 2.54 centimeters, we get 1 in 1=---2.54 cm This expression is called a unit factor. Since 1 inch and 2.54 centimeters are exactly equiv-alent, multiplying any expression by this unit factor will not change its value. The pin has a length of 2.85 centimeters. Multiplying this length by the appropriate unit factor gives 18 Chapter One Chemical Foundations 1 in 2.85 2.85.em X = -in = 1.12 in 2.54.em 2.54 Note that the centimeter units cancel to give inches for the result. This is exactly what we wanted to accomplish. Note also that the result has three significant figures, as required by the number 2.85. Recall that the 1 and 2.54 in the conversion factor are exact numbers by definition. EXAMPLE
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