Measurement - Fundamentals of Chemistry - page 52

Figure 2.8 Bathroom Scale
If a bathroom scale is not adjusted to
read zero when no load is on it, the
results it yields with a load may be
precise but probably not accurate.
For example, if it reads 2 lb with no
load, each time a person uses the
scale, it will probably read 2 lb
heavier than his or her true weight.
Practice Problem 2.35 A machine wraps sticks of butter with paper
that is printed with lines marking 1-oz portions. If the machine wraps the but-
ter one-eighth of an inch away from the correct position (Figure 2.9), the first
portion might be too small and the last too large. Having 12 sticks of butter all
wrapped by the same machine in the same way: (a) Will there be the same mass
of butter at the first mark each time? (b) Will each portion be 1 oz in mass?
(c) Is the wrapping more precise than accurate or vice versa?
The precision with which we measure must be indicated when we report a
measurement. When we use a measuring instrument, we must estimate to one
digit beyond the smallest scale division, if possible. For example, see Figure
2.10. If we measure the length of the bar with the top ruler, calibrated in cen-
timeters, we see that the bar is between 4 and 5 cm long and can estimate that
it extends 0.1 cm past 4 cm, for a reading of 4.1 cm. In contrast, if we use the
bottom ruler, calibrated in tenths of centimeters—that is, millimeters—we see that
4 TBSP = 1/4 CUP
8 TBSP = 1/2 CUP
Figure 2.9 Precise but Inaccurate Measurement
If the wrapping machine regularly places the wrapper too close to one end of each stick of butter, the end piece might be the
same weight in each stick but still be far from 1 tablespoon (TBSP).