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How big? . How small?. Significant Digits and Isotopic Abundance. How accurate? . Reminder: bring a calculator to class. Scientific notation. Read “Scientific Notation” on page 621 Complete the chart below. Decimal notation. Scientific notation. 127.

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How big

How big?

How small?

Significant Digits and Isotopic Abundance

How accurate?



Scientific notation
Scientific notation

  • Read “Scientific Notation” on page 621

  • Complete the chart below

Decimal notation

Scientific notation

127

1.27 x 102

0.0907

9.07 x 10–2

0.000506

5.06 x 10–4

2 300 000 000 000

2.3 x 1012


What time is it
What time is it?

  • Someone might say “1:30” or “1:28” or “1:27:55”

  • Each is appropriate for a different situation

  • In science we describe a value as having a certain number of “significant digits”

  • The # of significant digits in a value includes all digits that are certain and one that is uncertain

  • “1:30” likely has 2, 1:28 has 3, 1:27:55 has 5

  • There are rules that dictate the # of significant digits in a value (read handout up to A. Try A.)


Answers to question a
Answers to question A)

3

4

3

2

1

4

4

6

infinite

5

  • 2.83

  • 36.77

  • 14.0

  • 0.0033

  • 0.02

  • 0.2410

  • 2.350 x 10–2

  • 1.00009

  • 3

  • 0.0056040


Significant digits

  • Similarly, 6.54 = 6.545454…

Significant Digits

  • It is better to represent 100 as 1.00 x 102

  • Alternatively you can underline the position of the last significant digit. E.g. 100.

  • This is especially useful when doing a long calculation or for recording experimental results

  • Don’t round your answer until the last step in a calculation.


Adding with significant digits
Adding with Significant Digits repeats indefinitely. E.g. 9.6 = 9.6666…

  • Howfaris it fromToronto to room 229? To 225?

  • Adding a value that is much smaller than the last sig. digit of another value is irrelevant

  • When adding or subtracting, the # of sig. digits is determined by the sig. digit furthest to the left when #s are aligned according to their decimal.


Adding with significant digits1
Adding with Significant Digits repeats indefinitely. E.g. 9.6 = 9.6666…

  • Howfaris it fromToronto to room 229? To 225?

  • Adding a value that is much smaller than the last sig. digit of another value is irrelevant

  • When adding or subtracting, the # of sig. digits is determined by the sig. digit furthest to the left when #s are aligned according to their decimal.

  • E.g. a) 13.64 + 0.075 + 67 b) 267.8 – 9.36

13.64

267.8

+

0.075

9.36

+

67.

80.715

81

258.44

  • Try question B on the handout


B answers

i) repeats indefinitely. E.g. 9.6 = 9.6666…

ii)

iii)

83.25

4.02

0.2983

0.1075

+

0.001

+

1.52

B) Answers

83.14

4.02

1.82


Multiplication and division
Multiplication and Division repeats indefinitely. E.g. 9.6 = 9.6666…

  • Determining sig. digits for questions involving multiplication and division is slightly different

  • For these problems, your answer will have the same number of significant digits as the value with the fewest number of significant digits.

  • E.g. a) 608.3 x 3.45 b) 4.8  392

    a) 3.45 has 3 sig. digits, so the answer will as well

    608.3 x 3.45 = 2098.635 = 2.10 x 103

    b) 4.8 has 2 sig. digits, so the answer will as well

    4.8  392 = 0.012245 = 0.012 or 1.2 x 10–2

  • Try question C and D on the handout (recall: for long questions, don’t round until the end)


C d answers

i) repeats indefinitely. E.g. 9.6 = 9.6666…

7.255  81.334 = 0.08920

ii)

1.142 x 0.002 = 0.002

iii)

31.22 x 9.8 = 3.1 x 102 (or 310 or 305.956)

i)

6.12 x 3.734 + 16.1  2.3

22.85208 + 7.0 = 29.9

iii)

1700

ii)

0.0030 + 0.02 = 0.02

+

134000

iv)

33.4

135700 =1.36 x105

+

112.7

+

0.032

146.132  6.487 = 22.5268

= 22.53

C), D) Answers

Note: 146.1  6.487

= 22.522 = 22.52


Unit conversions
Unit conversions repeats indefinitely. E.g. 9.6 = 9.6666…

  • Sometimes it is more convenient to express a value in different units.

  • When units change, basically the number of significant digits does not.

    E.g. 1.23 m = 123 cm = 1230 mm = 0.00123 km

  • Notice that these all have 3 significant digits

  • This should make sense mathematically since you are multiplying or dividing by a term that has an infinite number of significant digits

    E.g. 123 cm x 10 mm / cm = 1230 mm

  • Try question E on the handout


E answers

i) repeats indefinitely. E.g. 9.6 = 9.6666…

1.0 cm = 0.010 m

ii)

0.0390 kg = 39.0 g

iii)

1.7 m = 1700 mm or 1.7 x 103 mm

E) Answers


Isotopic abundance
Isotopic abundance repeats indefinitely. E.g. 9.6 = 9.6666…

  • Read 163 – 165

  • Let’s consider the following question: if 12C makes up 98.89% of C, and 13C is 1.11%, calculate the average atomic mass of C:

Mass from 12C atoms

+

Mass from 13C atoms

12 x 0.9889

13 x 0.0111

+

= 12.01

11.8668

+

0.1443

  • To quickly check your work, ensure that the final mass fits between the masses of the 2 isotopes

  • Do the sample problem (page 165) as above and try questions 10 and 11 (page 166)


Isotopic abundance1

Mass from repeats indefinitely. E.g. 9.6 = 9.6666…35Cl

+

Mass from 37Cl

35 x 0.7553

37 x 0.2447

+

26.4355

+

9.0539

Mass from 39K

+

Mass from 41K

39 x 0.9310

41 x 0.0690

+

36.309

+

2.829

+

Mass from 38Ar

Mass from 40Ar

+

Mass from 36Ar

38 x 0.0006

40 x 0.9960

36 x 0.0034

+

+

+

0.0228

39.84

+

0.1224

Isotopic abundance

= 35.49

= 39.14

= 39.99


More practice
More practice repeats indefinitely. E.g. 9.6 = 9.6666…

  • Answer questions 1 – 3 on page 179. Your answers should have the correct number of significant digits.

For more lessons, visit www.chalkbored.com


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