Chemistry Reference
In-Depth Information
Figure 2.2
Various Measurements
(a) Measuring the diameter of a bowling ball by placing it between two parallel
boards and measuring the distance between the boards. (b) Measuring the
circumference of a bowling ball, whereby the diameter may be calculated using
(c) Measuring the volume of irregularly shaped, insoluble objects by
displacement of water. The volume of the copper shot is the total volume (shown)
minus the volume of the water before the shot was added.
c
d
.
(a)
(b)
(c)
designed to make calculations as easy as possible. In Section 2.4, we discuss
the accuracy and precision of measurements and how the precision should be
reported, using the proper number of significant digits. The concept of density,
considered in Section 2.5, not only is useful in itself, especially for identifying
substances, but also enables us to apply the concepts presented in previous sec-
tions. Finally, Section 2.6 briefly discusses temperature scales.
The measurement of energy will be discussed in Section 14.3, and kinetic
energy will be treated in Section 12.10.
2.1
Factor Label Method
Every measurement results in a number and a
unit.
Reporting the unit is just
as important as reporting the number. For example, it makes quite a bit of dif-
ference whether a pet is 6
inches
tall or 6
feet
tall! The units are an integral
part of any measurement, and from the outset, we must get used to stating the
units for every measured quantity and for every quantity calculated from mea-
sured data. Always use full spellings or standard abbreviations for all units.
In a great many cases, we can use the units as a clue to which operation—
multiplication or division—to perform in calculations with measured quantities.
The units of measurement can be treated as algebraic quantities in calculations.
For example, we can calculate the total wages of a student aide who has earned
9 dollars per hour for 30 hours of work, as follows:
Use units in reporting all
measurements and the results
of calculations using them.
Always use full spellings or
standard abbreviations for all
units.
Total wages
(hours worked) (hourly rate)
a
9 dollars
1 hour
30 hours
b
270 dollars
