Chemistry 3.1

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Chemistry 3.1

• Uncertainty in Measurements

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I. Accuracy, Precision, Error

• A. Accuracy how close a measurement comes to
  the true value.
• 1. Ex Throwing Darts
• true value bull's-eye

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• B. Precision how close a series of
  measurements are together.
• 1. Ex Throwing Darts

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• C. Pg. 64 Explanation

Poor Accuracy Good Precision
Poor Accuracy Poor Precision
Good Accuracy Good Precision
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• D. Error the difference between the accepted
  value and the experimental value.
• 1. Formula
• 2. Error ex. value accepted value
• error
• accepted value

Error
x 100
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• 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

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II. Significant Figures

• A. Def all digits known plus one estimated
  one.
• 1. Measurements must be recorded with
  significant figures.

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• 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)
• ? 200 ? (sig figs)
• ? 200.0 ? (sig figs)

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1
4
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• 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

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• 4. Scientific Notation Sig figs
• A. All s in scientific notation are counted
  as significant figures.
• B. Ex
• 3.0200 x 103 sig figs
• 2.77 x 106 sig figs

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3
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• 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)

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• 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)

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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