Ch. 3

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Ch. 3 Scientific Measurement

• Test Sept. 14th
• Workbook Units 1-3

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3.1 The Importance of Measurement
What types of measurements are made in
chemistry? Length, Temperature, Mass, Volume
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Qualitative v. Quantitative Measurements
Not all measurements give the same amount of info.

• Quantitative
• Require an instrument
• Have numbers and units
• Ex the diameter of the basketball is 31 cm, the
  persons temperature is 99.7o F

• Qualitative
• Descriptive
• Non-numerical
• Ex the basketball is brown, the person feels hot

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Scientific Notation
In chemistry, we use scientific notation b/c we
deal with extreme numbers.
36,000 3.6 x 104 and 0.0000073 7.3 x
10-6
coefficient
exponent
The exponent tells you how many places to move
the decimal and in which direction.
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Multiplication Division

• Multiply
• Multiply the coefficients
• Add the exponents
• Ex (3.0 x 103)(2.0 x 102) 6.0 x 105

• Divide
• Divide the coefficients
• Subtract the exponents
• Ex 3.0 x 105 / 2.0 x 102 1.5 x 103

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Addition and Subtraction
Before adding or subtracting in scientific
notation you must make the exponents the same.

• 7.1 x 10-2
• 0.50 x 10-2
• 6.6 x 10-2

• 7.1 x 10-2
• 5.0 x 10-3

Make sure the decimals are aligned.
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3.2 - Uncertainty in Measurements

• Accuracy how close a measurement is to the
  accepted (true) value
• Precision how close multiple measurements are
  to each other
• Error accepted value experimental value
• Error (error / accepted value) x 100

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Lets visualize a target
Accuracy Precision
Accuracy Precision
Accuracy Precision
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Percent Error Calculations
You measure the boiling point of water to be
99.3oC and the accepted value is 100.0oC Find
the error. 0.7 / 100 x 100 .7 Note Error
can be negative too so we use the absolute value
of it. You try Accepted value 7.5 m
Experimental value 7.9 m
5.3
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Significant Figures in Measurements

• Rules For Counting, Calculating, and Rounding
  Sig. Figs

http//chemed.chem.purdue.edu/genchem/topicreview/
bp/ch1/sigfigs.html

• Practicing with Sig. Figs

http//science.widener.edu/svb/tutorial/sigfigures
.html
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When are Digits Significant?
Non-zero digits are always significant. With
zeroes, the situation is more complicated a)
Zeroes placed before other digits are not
significant 0.046 has two significant digits.
b) Zeroes placed between other digits are always
significant 4009 kg has four significant digits.
c) Zeroes placed after other digits but behind a
decimal point are significant 7.90 has three
significant digits.
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When are Digits Significant? continued

• d) To avoid uncertainty, use scientific notation
  to place significant zeroes behind a decimal
  point
• 8.200 ? 103 has four significant digits
• 8.20 ? 103 has three significant digits
• 8.2 ? 103 has two significant digits

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2 Exceptions!!Measurements have unlimited number
of sig figs if..

• COUNTING IS INVOLVED.
• If you count how that there are 23 people in
  this classroom, then there are exactly 23 people,
  not 22.9 or 23.1
• This measurement can only be a whole number and
  has unlimited number of sig figs in the form of
  zeroes to the right of the decimal.

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Measurements have unlimited number of sig figs if

• 2) INVOLVES EXACTLY DEFINED QUANTITIES
• Usually associated with a system of measurement
• Ex) 60 minutes 1 hour
• By definition, there are exactly 60 minutes in
  an hour.
• Saying 60.0000000 does not change that there
  are 60 minutes in an hour!

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Sig Figs in Calculations

• Addition Subtraction the answer should be
  rounded to the same number of decimal places not
  digits as the measurement with the least number
  of dec. places
• EX Add 12.52m 349.0m 8.24m
• What is the correct rounded answer?

369.8 m
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Multiplication Division

• You need to round the number to the same number
  of sig figs as the measurement with the least
  number of sig figs.
• EX 7.55m x 0.34m

2.6 m2