Transcript Significant Figures and Density - University of North Carolina

CHM 101/102
Significant Figures and Density
General Chemistry 101/102
Laboratory Manual
University of North Carolina at Wilmington
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Purpose
 To gain an understanding of how to interpret
uncertainties in laboratory data.
 To become familiar with the use of common
laboratory equipment and glassware.
•
Safety Considerations
 There are no special safety considerations
associated with this laboratory exercise other than
washing your hands before leaving the lab.
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Accuracy vs. Precision
 Accuracy – a statement of how close a measured
value is to the true or accepted value.
 Precision – a statement of how reproducible the
measured results are.
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Significant Figures
 The value reported for a measurement is rounded
off so that it contains only the digits known with
certainty plus one uncertain one which is the last
digit.
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Significant Figure Rules
 To determine the number of significant figures in a
reported value, start at the left and count all digits
beginning with the first non-zero digit.
Examples: 0.003040 => 4 sig. fig.
10,200 => 5 sig. fig.
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Significant Figures in Calculations
 Addition & Subtraction: The number of decimal
places in the answer should be equal to the
number of decimal places in the value with the
fewest decimal places.
Example:
Laboratory Manual
12.0035 g
3.72 g
0.015 g
15.74 g
(4 decimal places)
(2 decimal places)
(3 decimal places)
(2 decimal places)
CHM 101/102
Significant Figures and Density
•
Significant Figures in Calculations
 Multiplication & Division: The number of significant
figures in the answer should be the same as the
value with the fewest significant figures.
Example:
12.36 mL
4 sig. fig.
x
2 sig. fig.
1.7 g
mL
=
21 g
2 sig. fig.
 Note: Definitions and counted numbers have an
unlimited number of significant figures.
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Procedure – Part 1 a & b
 Pour approximately 8 mL of water into a 50 mL
graduated cylinder. Record the measured volume
to the appropriate number of significant figures.
Repeat the procedure using a 10 mL graduate
cylinder.
read from the bottom of
the meniscus
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Procedure – Part 1 c & d
 Use a 10 mL graduated pipet to transfer 8 mL of
water to a 50 mL preweighed beaker. Reweigh
the beaker and use the mass of water and the
density of water to calculate the volume of water
transferred.
Laboratory Manual
CHM 101/102
Significant Figures and Density
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Procedure – Part 2
 Determine the identity of an unknown metal by
measuring its density (mass/volume). The volume
of the metal is determined by “displacement”.
Laboratory Manual
CHM 101/102
Significant Figures and Density
•
Procedure – Part 2
the difference
between these two
readings is the
volume of the metal
Laboratory Manual