Embed Size (px)
Citation preview
Errors in Measurements
Class measurements for the density of gold (in g/mL)
Group 1 17.8 Group 4 18.1
Group 2 18.0 Group 5 17.8
Group 3 17.9 Group 6 18.0
Errors in Measurements• Precision – Measurements close to
one another.
Group 1 17.8 Group 4 18.1
Group 2 18.0 Group 5 17.8
Group 3 17.9 Group 6 18.0
Class measurements are precise….
Errors in Measurements• Accuracy – Measurements close to
correct value.
Group 1 17.8 Group 4 18.1
Group 2 18.0 Group 5 17.8
Group 3 17.9 Group 6 18.0
The actual value for the density of gold is 19.3 g/mL.
So the class is not accurate…
[http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH01/FG01_08abcd.JPG]
Errors in Measurements
• Check your accuracy by calculating your Percent Error
100x accepted
accepted - measured error %
Errors in Measurements
• Accepted – What the “true” value is
• Measured – What you found in the experiment.
Errors in Measurements
• What is the percent error for the class data then?
100x 19.3
19.3 - 17.9
Percent Error = 7.25%
Class Avg.
Significant Digits• What is the reading for these
two volumes?
33.5 mL
30 mL
30.0 mL
[http://wps.prenhall.com/wps/media/objects/476/488316/Instructor_Resources/Chapter_02/FG02_07-30q.JPG]
Most AccurateCylinder?
65 mL vs. 5.50 mL
Significant Digits
Gas Station Analogy – Gas $2.50
17.585 gal = $43.96
17.59 gal = $43.98
17.6 gal = $44.00
18 gal = $45.00
Significant Digits
• Numbers that are “given” to you must be checked for their accuracy as well.
• 1.0400 vs. 10400 vs. 0.00104• Atlantic-Pacific Rule – Pg. 26
• All non-zero digits are significant• Zeros depend on the decimal point
1.0400
5 significant digits
10400
3 significant digits
0.00104
3 significant digits
6.02 x 1023
3 significant digits
Significant Digits• Multiply /
Divide• Use the
smaller number of significant digits for the answer.
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter01/Text_Images/FG01_07-05UN.JPG]
Significant Digits• Add /
Subtract• Line up
the decimal places then round answer.
[http://wps.prenhall.com/wps/media/objects/476/488316/Instructor_Resources/Chapter_02/FG02_04-06un.JPG]
