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Review Chapter Measurement and Calculations in Chemistry.

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Review Chapter Measurement and Calculations in Chemistry
Transcript

Review Chapter

Measurement and Calculations in

Chemistry

Section R.1

Units of Measurement

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• Quantitative observation consisting of two parts. number scale (unit)

Nature of Measurement

Measurement

• Examples 20 grams 6.63 × 10–34 joule·seconds

Section R.1

Units of Measurement

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The Fundamental SI Units

Physical Quantity Name of Unit Abbreviation

Mass kilogram kg

Length meter m

Time second s

Temperature kelvin K

Electric current ampere A

Amount of substance mole mol

Section R.1

Units of Measurement

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• Prefixes are used to change the size of the unit.

Prefixes Used in the SI System

Section R.1

Units of Measurement

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Prefixes Used in the SI System

Section R.2

Uncertainty in Measurement

6

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• A digit that must be estimated is called uncertain.

• A measurement always has some degree of uncertainty.

• Record the certain digits and the first uncertain digit (the estimated number).

Section R.2

Uncertainty in Measurement

7

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Measurement of Volume Using a Buret

• The volume is read at the bottom of the liquid curve (meniscus).

• Meniscus of the liquid occurs at about 20.15 mL. Certain digits: 20.15 Uncertain digit: 20.15

Section R.2

Uncertainty in Measurement

8

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Types of Errors

Every measurement has some uncertainty experimental error.

Experimental error is classified as either systematic or random.

Maximum error v.s. time required

Section R.2

Uncertainty in Measurement

9

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1) Systematic error

= Determinate error = consistent error

- errors arise: instrument, method, & person

- can be discovered & corrected

- from fixed cause, & is either high (+) or low (-) every time.

- ways to detect systematic error:

examples (a) pH meter (b) buret

Section R.2

Uncertainty in Measurement

10

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2) Random error = Indeterminate error

Is always present & cannot be corrected

Has an equal chance of being (+) or (-).

(a) people reading the scale

(b) random electrical noise in an instrument.

(c) pH of blood (actual variation: time, or part)

3) Precision & Accuracy

reproducibility

confidence of nearness to the truth

Section R.2

Uncertainty in Measurement

11

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Precision and Accuracy

Section R.3

Significant Figures and Calculations

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12Copyright © Cengage Learning. All rights reserved

1. Nonzero integers always count as significant figures. 3456 has 4 sig figs (significant figures).

Rules for Counting Significant Figures

Section R.3

Significant Figures and Calculations

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13Copyright © Cengage Learning. All rights reserved

• There are three classes of zeros.a. zeros that precede all the nonzero digits: do not count.

0.048 has 2 sig figs.

Rules for Counting Significant Figures

b. zeros between nonzero digits: always count. 16.07 has 4 sig figs.

c. zeros at the right end of the number: significant only if the number contains a decimal point. 9.300 has 4 sig figs. 150 has 2 sig figs.

Section R.3

Significant Figures and Calculations

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14Copyright © Cengage Learning. All rights reserved

3. Exact numbers have an infinite number of significant figures. 1 inch = 2.54 cm, exactly. 9 pencils (obtained by counting).

Rules for Counting Significant Figures

Section R.3

Significant Figures and Calculations

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15Copyright © Cengage Learning. All rights reserved

• Example 300. written as 3.00 × 102

Contains three significant figures.

• Two Advantages Number of significant figures can be easily indicated. Fewer zeros are needed to write a very large or very

small number.

Exponential Notation

Section R.3

Significant Figures and Calculations

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16Copyright © Cengage Learning. All rights reserved

1. For multiplication or division, the number of significant figures in the result is the same as the number in the least precise measurement used in the calculation.

1.342 × 5.5 = 7.381 7.4

Significant Figures in Mathematical Operations

Section R.3

Significant Figures and Calculations

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2. For addition or subtraction, the result has the same number of decimal places as the least precise measurement used in the calculation.

Significant Figures in Mathematical Operations

Corrected

23.445

7.83

31.2831.275

Section R.3

Significant Figures and Calculations

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18

Logarithms and Antilogarithms

• The base 10 logarithm of n is the number a, whose value is such that n=10a:

• The number n is said to the antilogarithm of a.

P.64

Section R.3

Significant Figures and Calculations

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19

• In converting a logarithm to its antilogarithm, the number of significant figures in the antilogarithm should equal the number of digits in the mantissa.

P.65

Section R.3

Significant Figures and Calculations

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20

[H+]=2.010-3

pH=-log(2.010-3) = -(-3+0.30)=2.70

antilogarithm of 0.072 1.18

logarithm of 12.1 1.083

log 339 = 2.5301997… = 2.530

antilog (-3.42) = 10-3.42 = 0.0003802 = 3.8x10-4

Logarithms and Antilogarithms

Section R.3

Significant Figures and Calculations

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21Copyright © Cengage Learning. All rights reserved

Concept Check

You have water in each graduated cylinder shown. You then add both samples to a beaker (assume that all of the liquid is transferred).

How would you write the number describing the total volume?

3.1 mL

What limits the precision of the total volume?

Section R.7

Classification of Matter

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The Organization of Matter


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