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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 Spring '10
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 Organic chemistry, Valence, Conversion of units, Inch, centimeters, unit factor

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