Chem 108, Long
Measured and Exact Numbers
Measured Numbers
All of these numbers have uncertainty in the last digit on
the right (ie. the last digit on the right was estimated.
Significant figure rules must be used in writing these
numbers and in rounding off answers to problems in which
these numbers were used.
Types:
1. Numbers measured with a measuring device
2. Numbers estimated by some other means
3. Numbers in a conversion factor showing a relationship
between two different measurement systems (SI and English).
Except that 2.54 cm = 1 inch was redefined as exact
Exac t Numbers
There is no uncertainty in these numbers.
Significant figure rules are not applied to these numbers,
either when writing them or when using them in problems.
Types:
1. Counted numbers (where the numbers are small enough to
accurately count)
2. Numbers in a conversion factor showing the relationships
between units within the same measurement system (defined
conversions)
Note: You do not apply significant figure rules to a 1 on one
side of a conversion factor. The 1 in a conversion factor used
in a problem is considered to be as accurate (have as many
significant figures) as the number to which it is related on the
other side of the factor.