Chapter 3.2 Uncertainty in Measurements!
Objectives: You should be able to!
1)  Distinguish among the accuracy, precision, and
error of a measurement.!
2) 
Identify the number of significant figures in a
measurement and in the result of a calculation.!
Accuracy, Precision, and Error"
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Percent error!
Error = accepted value - experimental value where the
accepted value is the correct value minus the value measured
in the lab.!
!  Errors can be + or -!
% error = | error | x 100%!
!accepted value!
Chapter 3.2 Uncertainty in Measurements!
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Accuracy, Precision, and Error!
Measurements need to be correct and reproducible.!
!  Accuracy is a measure of how close a measurement comes
to the true value.!
!  Precision is a measure of how close a series of
measurements are to one another.!
!  Figure 3.4 page 54!
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Significant Figures in Measurements!
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Significant figures are those digits that are known plus a last
digit that is estimated.!
Here are the rules.!
Every nonzero digit in a reported measurement is assumed to
be significant.!
!  35
!
!3.5 !
!.35!
Zeros between nonzeros are signigicant.!
!  3005
!
!3.005 !
!.3005!
Significant Figures in Measurements!
! 
! 
Leftmost zeros in front of nonzero digits are not significant!
!  035
!
!03.5 !
!.035!
!  If you use scientific notation you will have the correct
number of significant figures.!
!  3.5 x 101
!3.5 x 10 0
!3.5 x 10 -2!
Zeros at the end of a number and to the right of a decimal
point are always significant.!
!  350
!
!3.50 !
!.0350!
Significant Figures in Measurements!
! 
! 
Unlimited significant figures occur when…!
!  You count. There are 24 students in the room. Not parts of
students. 24.0000000000000000…!
!  Exact defined quantities. 60 minutes in an hour.!
!  60.00000000000000000000000…!
Practice problems page 58 # 5!
!  0.05730 meters!
!  8765 meters!
!  0.00073 meter!
!  40.007 meters!
Significant Figures in Measurements!
! 
Zeros at the rightmost end of a measurement that lie to the left
of an understood decimal point are not significant if they serve
as place holders.!
!  350
!7000 !
!57810!
!  Using scientific notation would eliminate the confusion.!
!  3.5 x 101
!7.000 x 10 3 !
!5.781 x 104 !
Significant figures in calculations!
! 
Rounding!
! 
You can only be as accurate as your least accurate
measurement.!
! 
5.4 x 4.56 = 24.624!
!
!Correct answer?!
Addition and subtraction!
!align the decimal points and use the number of least
significant figures.!
Mini Lab!
! 
Accuracy and Precision page 62!
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Homework Practice problems 5-10 assorted pages!
! Section review page 62 # s 13-16!
!Chem ASAP 3.2!