1 scientific method & measurement chemistry. scientific method observation hypothesis experiment...

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Scientific Method & Measurement

Chemistry

Scientific Method Observation

Hypothesis

Experiment Theory Law

If hypothesis is false, propose new hypothesis.

Must repeat several times.

Models, Laws & Theories

model: visual, verbal and/or mathematical explanation

of data; can be

-tested

-used to make predictions

theory: explanation based on supported hypothesis

-broad principle of nature supported over

many years

-can be modified

-can lead to new conclusions

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Models, Laws & Theories

law: describes something known to happen without

error

-doesn’t explain why it happens

-there are no exceptions

-several scientists come to the same conclusion

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Data

When making observations or gathering information, we separate the data into 2 types:1. qualitative-uses the 5 senses

-physical characteristics

2. quantitative-numerical data -measurable

Variables

There are 2 types of variables when doing an experiment:1. independent: variable you change2. dependent: variable that changes due to a change in the independent variable.

It is also important to have controls, or standards for comparison.

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Significant Figures

significant figures: number of all known digits in a measurement plus one estimated digit.

-allows more precision in measurement

-not all measuring devices show the same precision

Example: In the following measurement, what are the known values and what is the estimated value?

16.25 mL

known = 162

estimated = 5

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Significant Figures-Rules

The easiest way to determine significant figures of a given number is by using the Pacific/Atlantic rules.

1. Decimal point PRESENT, start from the PACIFIC. -Begin counting on the left hand (Pacific) side of the number. Move toward the right and start with the first nonzero number. 306.4000 has 7 significant figures

0.00040 has 2 significant figures

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Significant Figures-Rules

2. Decimal point ABSENT, start from the ATLANTIC.

-Begin counting on the right hand (Atlantic) side of

the number. Move toward the left and start with

the first nonzero digit.

1200 has 2 significant figures

1207 has 4 significant figures

Zeros that act as placeholders are not significant:

0.00040and 1200

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Significant Figures Practice 1

Determine the number of significant figures in the following numbers.

1) 0.02 4) 501.0 7) 0.0005

2) 0.020 5) 5000 8) 0.1020

3) 501 6) 5000.

Determine the number of significant figures in the following numbers.

9) 8040 12) 2.00x102 15) 0.000410

10) 0.0300 13) 0.90100

11) 699.5 14) 90100

Copy the following questions and answer.

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Significant Figures and Rounding

Suppose you are asked to find the density of an object with a m=of 22.44 g and whose V=14.2 cm3.

Using a calculator, you get 1.5802817, which has 8 significant figures. Does this answer make sense?

No. The mass only has 4 sig figs and the volume has 3. Your answer would be more precise than the starting information.

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Significant Figures and Rounding

How would you correctly round this?

By using the starting data with the fewest sig figs (when multiplying/dividing), which is 3: 1.58 g/cm3

-when adding/subtracting, your answer will have the smallest number of decimal places based on the starting information.

3.12 m + 3.2 m = 6.32 m = 6.3 m

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Significant Figures Practice 2

Perform the following operations expressing the answer in the correct number of significant figures.

1) 1.35 m x 2.467 m

2) 1035 m2 ÷ 42 m

3) 12.01 mL + 35.2 mL + 6 mL

4) 55.46 g – 28.9 g

5) 1.278x103 m2 ÷ 1.4267x102 m

Round all numbers to four significant figures.

6) 84791 kg 8) 0.0005481 g 10) 136758 m

7) 38.5432 g 9) 4.9356 mL

SI Units and Derived Units

      Unit Quantity Symbol Unit Abbrev.

length l meter m mass m kilogram kg

time t second s

temperature T Kelvin K

amount of substance

n mole mol

electric current

I ampere A

luminous intensity

Iv candela cd 14

The SI base unit is the unit in a system of measurements that is based on an object or event in the physical world.

Temperature

There are three possible temperature scales:

1.Celsius-based on metric system

-based on temp when water freezes and

boils

2.Kelvin-SI Unit

-based on the idea of absolute zero, the

lowest possible theoretical temperature

-will discuss more in Ch 14 (Gas Laws)

3. Farenheit-what we are used to using

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Converting Temperature

1. Celsius to Kelvin / Kelvin to Celcius

TK = TC + 273.15

TC = TK - 273.15

2. Celsius to Farenheit / Farenheit to Celsius

TC = (TF -32oF)5oC

9oF

TF = TC 9oF + 32oF

5oC

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SI Units and Derived Units

Not all quantities can be measured with base units.

derived unit: unit that is defined by a combination of base units

-volume: space occupied by an object; unit is the liter, L, for liquids and gases, or cubic centimeter, cm3, for solids

V = l x l x l

-density: ratio of the mass of an object to its volume; unit is g/mL or g/cm3 since 1 mL = 1cm3

D = m/V

PrefixesPrefix Symbol Meaning Multiple of Base Unit 10n

yotta- Y septillion 1,000,000,000,000,000,000,000,000 1024

zetta- Z sextillion 1,000,000,000,000,000,000,000 1021

exa- E quintillion 1,000,000,000,000,000,000 1018

peta- P quadrillion 1,000,000,000,000,000 1015

tera- T trillion 1,000,000,000,000 1012

giga- G billion 1,000,000,000 109

mega- M million 1,000,000 106

kilo- k thousand 1000 103

hecto- h hundred 100 102

deca- da ten 10 101

base        deci- d tenth 0.1 10-1

centi- c hundredth 0.01 10-2

milli- m thousandth 0.001 10-3

micro- millionth 0.000 001 10-6

nano- n billionth 0.000 000 001 10-9

pico- p trillionth 0.000 000 000 001 10-12

femto- f quadrillionth 0.000 000 000 000 001 10-15

atto- a quintillionth 0.000 000 000 000 000 001 10-18

zepto- z sextillionth 0.000 000 000 000 000 000 001 10-21

yokto- y septillionth 0.000 000 000 000 000 000 000 001 10-24

Converting Between Prefixes

Dimensional analysis is a method of problem-solving that focuses on the units used to describe matter.

A conversion factor is a ratio of equivalent values used to express the same quantity in different units.

-they change the units of a quantity without changing its value

-ratio of units, such as 1 km

1000m

-set up so the units you don’t need cancel out

48 m x 1 km = 0.048 km

1000 m

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Dimensional Analysis

It is common in scientific problems to use dimensional analysis to convert more than one unit at a time.

What is the speed of 550 m/s in km/min?

1.Convert m to km

2.Convert s to min

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Dimensional Analysis

Sometimes we need to convert from metric to standard (and vice versa).

-some of these common conversions you will need to know are:

1 cm3 = 1 mL 60 s = 1 min

1 in = 2.54 cm 60 min = 1 hr

1 ft = 12 in

Practice:

1.152 cm = ____ m 3. 152 s = ____ hr

2.42.5 in = ____ ft 4. 15 mL = ____ cm3

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Accuracy and Precision

accuracy: how close a measured value is to an accepted value.

precision: how close a series of measurements are to one another.

-may not be accurate

Example: For the following data, the actual density value is 1.59 g/cm3.

Density collected by Three Students.

  A B C

T1 (g/cm3) 1.54 1.40 1.70

T2 (g/cm3) 1.60 1.68 1.69

T3 (g/cm3) 1.57 1.45 1.71

Avg. (g/cm3) 1.57 1.51 1.70

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Percent Errorpercent error: ratio of the difference in the measured

value and accepted value divided by the accepted value multiplied by 100

% error = │measured value – accepted value│ x 100 accepted value

Ex: Calculate the % error of Student A’s Average Data. % error = │1.57 g/cm3 – 1.59 g/cm3 │ x 100

1.59 g/cm3

= │-0.02 g/cm3 │ x 100 1.59 g/cm3

= 0.02 g/cm3 x 100 1.59 g/cm3

= 1 %

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Accuracy & Precision Practice

Density collected by Three Students.

  A B C

T1 (g/cm3) 1.54 1.40 1.70

T2 (g/cm3) 1.60 1.68 1.69

T3 (g/cm3) 1.57 1.45 1.71

Avg. (g/cm3) 1.57 1.51 1.70

Calculate the percent error for each of the three students (A, B, C). The accepted value is 1.59 g/cm3)

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Graphing-You Try

Graph the data set A for T1, T2, and T3 using the rules you know.

Density collected by Three Students.

  A B C

T1 (g/cm3) 1.54 1.40 1.70

T2 (g/cm3) 1.60 1.68 1.69

T3 (g/cm3) 1.57 1.45 1.71

Avg. (g/cm3) 1.57 1.51 1.70

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Graphing

In chemistry, we mainly deal with line graphs.

A graph is used to reveal patterns by giving a visual representation of data.

a. must know the independent (x axis) and dependent variable (y axis) b. determine the range of data that needs to be plotted for each axis: try to take up at least ¾ of the paper -use a pencil and ruler c. number and label each axis: don’t forget the units d. plot the points and draw a line of best fit -curved or straight e. title the graph

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Graphing Practice

Complete the problem-solving lab at the bottom of page 44 in your textbook. Answer the Analysis and Thinking Critically questions on the back of the graph.

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How speed affects stopping distance

102030405060708090

100

10 15 20 25 30

speed (m/s)

sto

pp

ing

dis

tan

ce

(m

)

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Scientific Notation

Values in science are often very large or very small, requiring a lot of zeros.

-ex: the distance between Earth and Neptune is

4,600,000,000,000 m apart and the speed of light

is 300,000,000 m/s.

this is a lot of zeros to keep track of.

Q: What do scientists do?

A: they use scientific notation, a short hand method of

writing extremely large or small numbers, to make

their calculations easier.

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scientific notation is a value written as a simple number multiplied by a power of 10.

Power of 10 equivalents:

104 = 10,000

103 = 1000

102 = 100

101 = 10

100 = 1

10-1 = 0.1

10-2 = 0.01

10-3 = 0.001

10-4 = 0.0001

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Writing Scientific Notation

1. Write the first 2 or 3 digits as a simple number with only one digit to the left of the decimal point.2. Count the number of decimal places you move the decimal. This will give you your power of 10. -If you move the decimal to the left the power of ten

will be positive. -If you move the decimal to the right the power of ten

will be negative.3. If you must adjust the decimal: -if you move it to the left, you add to the exponent

-if you move it to the right, you subtract the exponent

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Dividing with Scientific Nototation Example

Lets calculate the time it takes for light to travel from Neptune to Earth. The speed of light is 3.0x108m/s and the distance from Neptune to Earth is 4.6x1012m.

-Use the formula v = d/t

-Rearrange to solve for t: t = d/v

-d = 4.6x1012m, v = 3.0x108m/s, t = ?

- t = 4.6x1012m = 1.5x104s (no adjustment)

3.0x108m/s

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Dividing with Scientific Notation PracticeYou may not use a calculator.

Convert the following into scientific notation.

1. 0.000521 2. 1526000

3. 126580 4. 102300000

Convert the following into common form.

5. 2.35x10-2 6. 6.458x104

7. 4.2512x10-8 8. 1.520x102

Solve the following:

9. 2.70x105 ÷ 3.0x102

10. 2.0x104 ÷ 5x102

11. 8.0x106 ÷ 4.00x103

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Multiplying with Scientific NotationExample

If it takes 2.7 x 1023 seconds for light to travel from one planet to another, how far apart are the planets? Remember light travels at a speed of 3.0 x 108 m/s.

-Use the formula v = d/t.

-Rearrange to solve for d: d = vt

-d = ?, v = 3.0 x 108 m/s

d = vt = (2.7 x1023 s) (3.0 x 108 m/s)

= 8.1 x 1031 m (no adjustment necessry)

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Multiplying with Scientific Notation Practice

You may not use a calculator.

Review with dividing:

1. 1.2x103 ÷ 2.4x104

2. 4.6x10-3 ÷ 2.3x10-5

3. 6.02x105 ÷ 2.0x102

Multiplying:

4. (1.2x103)(2.4x104)

5. (4.6x10-3)(2.3x10-5)

6. (6.02x105)(2.0x102)

7. (2.70x105)(3.0x10-2)

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Cumulative Scientific Notation Practice

You may not use a calculator.1. Write the following measurement in scientific notation. a. 37,500,000,000,000,000,000,000 m b. 0.000012 kg2. Write the following values in long (standard) form a. 4.5 x 103 grams b. 3.115 x 10-6 km3. Multiply. a. (3.5 x 1012)(2.2 x 105) b. (7.5 x 10-3)(1.2 x 10-2)4. Divide a. 3.5 x 1019 b. 4.6 x 10-3

1.2 x 107 2.1 x 10-7

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Combined Measurement Practice

Show all work, including units!!Metrics: Convert the following:

1) 35 mL = ____ L 2) 0.005 kg = ____ g

Dimensional Analysis: Convert the following:

3) 3500 s = ____ hr 4) 4.2 L =_____ cm3

Scientific Notation: Convert to scientific notation:

5) 0.005 6) 505 7) 750600

Scientific Notation: Convert to standard notation:

8) 1.5x103 9) 3.35x10-6

Calculations: using Scientific Notation

10) (1.5 x 103)(3.5x105) 11) (3.45x10-3)/(1.2x 10-2)

12) (7.6x10-3)(8.2x107 ) 13) (6.8x107)/(2.2x10-5)