Regents Chemistry

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Regents Chemistry

• Significant Figures

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Five-minute Problem

• How many significant figures are in the
  following (write the number and answer)
• 125
• 1.256
• 0.0000004567
• 0.00300
• 1.004623

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Significant FiguresWhy?

• Allow us to make an accurate measurement!
• Contain certain numbers
• and one uncertain number

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Certain Numbers

• Same regardless of who made the measurement
• Actual divisions marked on instrument
• Example Ruler, beaker

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Uncertain Numbers

• Are an estimate
• Vary by person and trial
• For example estimate with a ruler, beaker

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Significant Figures Include...

• All certain numbers and one uncertain number
• For example 8.55 cm is actually
• 8.55 0.01

-
The last digit is not actually on the ruler you
must make an estimate!
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Rules for Counting Sig. Figs.

• 1. Nonzero integers - always count
• ex 1322 has four significant figures
• 2. Zeros
• Leading Zeros - precede all nonzero digits
• and do not count! Ex 0.00025
• Captive Zeros - fall between nonzero digits
• and always count! Ex 1.008
• Trailing Zeros - zeros at end of number Ex. 100.
  vs. 100
• Significant only if the number contains a
  decimal

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Rules for Counting Sig. Figs.

• 3. Exact Numbers - have an unlimited
• amount of significant figures
• 2 Kinds
• Describe something50 cars, 25 bugs
• By definition 1 in 2.54 cm

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Rounding Numbers and Sig Figs

• Less than 5
• Equal to/more than 5

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Sig Figs in Calculations

• Multiplication and Division
• Smallest amount of significant figures
  determines..
• Ex 4.56 x 1.4 6.384
  6.4
• Ex 8.315

Round off
(3)
(2)
(4)
Round off
2.79 x 10-2
0.0279027

298
(3)
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Sig Figs in Calculations

• Addition/Subtraction - the number with the
  smallest amount of decimal places limits amount
  of significant figures
• Ex 10.2 1.7 4.13 5.765
• 21.795
• Ex 25 15.2 125.5 165.7
  166

Round off
21.8
Round off