Transcript Chapter 3 Lesson 2

Chemistry 3.1
Uncertainty in
Measurements
I. Accuracy, Precision, & Error
A. Accuracy – how close a measurement
comes to the “true value”.
1. Ex: Throwing Darts
true value = bull's-eye
B. Precision – how close a series of
measurements are together.
1. Ex: Throwing Darts
C. Pg. 64 Explanation
Poor Accuracy
Good Precision
Good Accuracy
Good Precision
Poor Accuracy
Poor Precision
D. Error – the difference between the
accepted value and the experimental
value.
1. Formula –
2. Error = ex. value – accepted value
| error |
accepted value
E. Examples
1. In class you determine the melting
point of salt is 755 deg C. The actual
value is 805 deg C. What is your
percent error?
 [|755 - 805| / 805] x 100 =
 6.2% error
II. Significant Figures
A. Def – all digits known plus one
estimated one.
1. Measurements must be recorded with
significant figures.
2. Rules (pg.66)
-All other numbers are significant
-zeros may or may not be significant
-leading zeros are not significant
 0.02
1 (sig fig)
-captive zeros are significant
 0.0203
3 (sig figs)
-trailing zeros following the decimal
point are significant
 0.02030
? (sig figs) 4
 200
? (sig figs) 1
 200.0
? (sig figs) 4
3. Rounding with Sig Figs
-Express the following #’s to 3 sig figs
 421798.076
= 422,000
 0.00099985
= .00100
1
= 1.00
 8222
= 8,220
 0.42
= .420
4. Scientific Notation + Sig figs
A. All #’s in scientific notation are
counted as significant figures.
B. Ex:
3.0200 x 103 = 5 sig figs
2.77 x 106
= 3 sig figs
5. Adding and subtracted
A. The answer must not contain any sig
figs beyond the place value common to
all #’s
B. Ex:
4.8
+ 2.015
6.8
(not 6.815)
6. Multiplication and Division
A. The answer must not contain more
sig figs than the least # of sig figs.
B. Ex:
3.1
x 4.01
12
(not 12.431)
In Class Problems
1. How many sig figs?
-123 meters
-30.0 meters
-40,506 kg
-6.455 x 103 kg
2. 3.45 + 9.001
and 4.22 - 9.0
3. 3.4 x 5.345
and 10.7 / 12.75
4. 6.33 x 103 + 5.1 x 104