Chapter 5.4
Measured Numbers (Certain and Uncertain)
Mrs. Baldessari
Chemistry
Objective: You will explain why
there is uncertainty in measurement
and need for sig figs.
Measured Numbers
When you use a measuring tool is used to
determine a quantity such as your height or
weight, the numbers you obtain are called
measured numbers.
Reading a Meterstick
. l2. . . . I . . . . I3 . . . .I . . . . I4. .
cm
First digit (known)
=2
2.?? cm
Second digit (known)
= 0.7
2.7? cm
Third digit (estimated) between 0.05- 0.07
Length reported
=
2.75 cm
or
2.76 cm
or
2.77 cm
Known + Estimated Digits
 Known digits 2 and 7 are 100% certain
 The third digit 6 is estimated (uncertain)
 In the reported length, all three digits
(2.76 cm) are significant including the
estimated one
Learning Check
. l8. . . . I . . . . I9. . . .I . . . . I10. .
What is the length of the line?
1) 9.2 cm
2) 9.22 cm
3) 9.23 cm
How your answer compare with your
neighbor’s answer? Why or why not?
cm
Solution
. l8. . . . I . . . . I9. . . .I . . . . I10. .
cm
Estimate to the hundreth’s place (0.01 cm)
2) 9.22 cm
3) 9.23 cm
The estimated digit may be slightly different.
Both readings are acceptable.
Learning Check
l5. . . . I . . . . I6. . . .I . . . . I7. .
What is the length of the line?
1) 6.0 cm
2) 6.06 cm
3) 6.60 cm
cm
Solution
l5. . . . I . . . . I6. . . .I . . . . I7. .
cm
Estimate to the hundreth’s place (0.01 cm). The
reading is between 6.0 and 6.1. You may have
estimated 6.07 cm as well.
2) 6.06 cm
Zero as a Measured Number
. l3. . . . I . . . . I4 . . . . I . . . . I5. .
cm
What is the length of the line?
First digit
4.?? cm
Second digit
4.5? cm
Last (estimated) digit is
4.50 cm
(not to the left or right of .5)
Exact Numbers
Obtained when you count objects
2 soccer balls
1 watch
4 pizzas
Obtained from a defined relationship
1 foot = 12 inches
1 meters = 100 cm
Not obtained with measuring tools
Learning Check
A. Exact numbers are obtained by
1. measuring
2. counting
3. definition
B. Measured numbers are obtained by
1. measuring
2. counting
3. definition
Solution
A. Exact numbers are obtained by
2. counting
3. definition
B. Measured numbers are obtained by
1. Using a measuring tool
Learning Check
Classify each of the following as an exact (1)
or a measured (2) number.
A.___Gold melts at 1064°C
B.___1 yard = 3 feet
C.___A red blood cell with diameter 6 x 10-4 cm
D.___There were 6 hats on the shelf
E.___A can of soda contains 355 mL of soda
Solution
Classify each of the following as an exact (e) or a
measured(m) number. Give reason.
A. m Requires a thermometer(measuring tool)
B. e From a definition in U.S. system
C. m Need measuring tool to determine
D. e Counted the hats
E. m Measured