Numbers and the SI Units
Numbers and the SI Units
If a student is to be successful in chemistry, or for that matter any science with quantitative aspects, the
student must have a clear and complete knowledge and understanding of numbers.
That is, how accurately
do I know the number and what does the number represent?
Measurements are the means by which scientists gather quantitative information.
The beginning student
generally recognizes the importance of making a careful measurement, but usually fails to recognize that
great care must also be exercised in recording the measurement.
Hence, it must be recognized that there are
parts to any measurement, the number and the label (or unit) associated with it. Both must be
stated clearly when quoting a measurement.
The numerical portion of a measurement relates the actual value obtained and the accuracy to which it is
known. In your own work, you will always want to convey, exactly, the accuracy to which a value is known.
This is true because, even if the exact degree of accuracy is not important to you in your work, it may be
important to someone else who, at some later time, may try to use your results for a different purpose.
There are two distinct classes of numbers. They are:
Exact numbers (known with absolute or infinite accuracy), and
Measured numbers (known with relative or finite accuracy).
The exact numbers can be divided into two types.
as the "9" in 9 baseballs, or the "3" in 3 eggs, and
as the "12" in 12 inches equal a foot, or
the "16" in 16 ounces equal a pound.
There is no uncertainty in either type of exact number because it is not possible, for example, to have a
fractional number of baseballs (that is, 9.1 baseballs is not possible) or anything other than exactly 16 ounces
in a pound.
Therefore, it must be kept in mind that the accuracy of any calculation involving exact numbers
is limited only by the accuracy of those numbers which are not exact numbers, namely those numbers which
are derived from measurements.
One could argue that counting 9 000 000 baseballs makes that a counted
number and therefore exact.
However, in this case (because the number is so large), the odds are great that
some error was made in the counting. Therefore, one must use some judgement in deciding the accuracy of
any such number.
Conversion factors within one system of measurement are exact; they are defined numbers. (Conversion
factors between different systems of measurement usually need to be compared by some measurement and,
therefore, are approximate. However, exceptions do exist--when one system is defined in terms of the other
E.g., 1 inch
25.4 millimeters, and 1 calorie
Both of these conversions are now
Therefore, it is easy to see why it is desirable to do all work within one system.