Transcript accuracy and precision 1 chem 2

Accuracy and Precision
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A MEASURE
of your
SUCCESS!
IT’S TRUE!
 Accuracy:
~ indicator of the closeness of a
measurement to its ‘true’ or ‘accepted’
value
 Dependent upon:
~ how carefully the measurement is taken
~ nature of the instrumentation used
Right On!
 Precision:
~ describes the agreement among
several results measured in the same
way
~ ‘reproducibility’ of measurements
Tell Me!
High or Low accuracy and precision
Low accuracy
High accuracy
Low precision
High precision
Low accuracy
High precision
Lab things to consider
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Procedures v. Data table
Using the balance
Volume v. volume
What is a meniscus?
Collecting data v. analyzing data
Does everyone have a calculator?
Are all those zeros necessary?
Significant figures (or digits)
• Things to consider:
D = m/V
m= 14.3216 g
D=?
V = 2.00 cm3
Exact numbers (no uncertain digit)
• Counted numbers (students in the class)
• Numerical relationships (1 km = 1000 m)
Uncertain digit
-Last digit in a
measurement that can
be read with
reasonable reliability
Deviation – Page 33
Inexact numbers (last digit is always uncertain digit)
• Result from measurement ( 25.3 mL in grad. Cylinder)
• Depend upon what measuring device is being used
RULES TO LIVE BY…
 All nonzero digits are significant
96 g
? Sig figs.
61.4 cm
? Sig figs.
 One or more final zeros used after the
decimal point are always significant
4.720 mL
? Sig figs.
8.00 g
? Sig figs.
 Zeros between other significant digits are
always significant
5.03m
? Sig figs.
3.008 g
? Sig figs.
 Zeros used solely for spacing the decimal point
are not significant. (place holders)
.00054 cm
? Sig figs.
0.01354 mg
? Sig figs.
1. Addition and Subtraction:
~ the answer must be rounded to the same number
of digits to the right of the decimal point as there
are in the measurement with the smallest number of
digits to the right of the decimal
EX. 2.89 m + 0.0043 m = 2.8943 m
= 2.89 m (correct # of sig figs)
2. Multiplication and Division:
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the product or quotient should be rounded to
the same number of significant figures as in the
measurement with the fewest total number of
significant figures
Ex. 3.5293 g x 34.2 g
= 120.70206 g2
= 121 g2 (correct # of sig figs)