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Welcome to Chemistry!with Mrs. Strain Rm. 403
Do Now: Find your seat and lab pass. If you have a smart phone, please take it out.
HWK: 1. Familiarize yourself with the information we
went over today.2. Review safety information – safety quiz on lab
day3. Start “Intro to WebAssign”: due next
Wednesday4. Obtain supplies & Signature Sheet by Monday.5. Review algebra for math assessment
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Taking Measurements in ChemistryAccording to the Scientific Method
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The Scientific Method
Scientific Method: logical approach to solving problems by…a. Observing & collecting datab. Formulating hypothesesc. Testing hypothesesd. Formulating theoriese. Publishing results
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Matter can be described in two ways: Qualitatively Quantitatively
Two Types of Measurements
Qualitative (think “quality”): observations using words
Quantitative (think “quantity”): observations using numbers and units.
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Here’s what I am hoping to see…
Qualitative observations: States of matter Color Texture Smell Viscosity
Quantitative observations: Amount of substances present
Step by step procedure!
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Here’s what I don’t want to see…
Opinionated language “I feel” “I like”
Non-specific wording “sort of…”, “lots of…”, ”kinda”
Descriptions that sound like a kindergartener wrote them “It was all bouncy and …” describing something as “chunky”
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Studying a System
System: specific portion of matter in a given region of space that has been selected for study Microscope or macroscopic
Variable: any condition that changes during an experiment Independent: value being manipulated Dependent: result
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Studying a System
Experimental Control: conditions that remain constant throughout (i.e. don’t change) Often many controlled portions of
system
Model: Explanation of how phenomena occur and how data or events are related
Theory:
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Taking Measurements in ChemistryGraphing Measurements
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Time in Seconds
Crystal Growth in
centimeters
6 .5
9 .9
15 1.7
23 2.2
T GrowthDirect Relationship
Independent Variable
Dependent Variable
Title Appropriate scale Axis labeled“Best fit” line
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Directly Proportional Relationships
Drag picture to placeholder or click icon to add
• When 2 quantities divided by each other gives a constant value
• K (constant value) = Y/X
• Ex: Density
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Inversely Proportional Relationships
Drag picture to placeholder or click icon to add
• When 2 quantities multiplied by each other gives a constant value
• K = X Y
• Ex: Boyle’s Law K = PV
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Taking Measurements in ChemistryCh. 2 The SI or Metric System
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Do Now: Test your Metric System “With-it-ness”
For each of the measurements on your worksheet, decide the appropriate quantity that should be assigned to it.
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The SI System
Around 1793, scientists all over the world began to agree upon a single measurement system called
Le Systeme International d’ Unites or SI System
7 base units
The idea was to create a unifying system of weights and measurements
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Quantity Unit Symbol
Length Meter mMass Kilogram kgTime Second sTemperature
Kelvin K
Amount of a substance
Mole mol
Electric current
Ampere A
Luminous intensity
Candela cd• Crash Course: Units• Where’s volume??
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Combinations of base units
Volume: amount of space taken up by an object Derived SI unit is cubic meter, m3 More often we use cm3 = mL
Density: ratio of mass to volume g/cm3 of g/mL or g/L Does not change for a given substance
Derived Units
m
D V
D = m V
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Other Derived Units
Quantity Unit Symbol
Derivation
Area square meter
m2 Length x width
Molar Mass
grams per mole
g/mol Mass / amount
Energy joule J Force x length
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Metric Prefix
Symbol
Meaning Scientific
Notation
mega M Million / 1,000,000 1 x 106
kilo k Thousand / 1,000 1 x 103
hecta h Hundred / 100 1 x 102
deka da Ten / 10 1 x 101
Base Unit
deci d Tenth / .1 1 x 10-1
centi c Hundredth / .01 1 x 10-2
milli m Thousandth / .001 1 x 10-3
micro Millionth / .000 001 1 x 10-6
nano n Billionth / .000 000 001 1 x 10-9
pico p Trillionth / .000 000 000 001
1 x 10-12
Larg
er
qu
an
titi
es
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Using SI prefixes: Number Line MethodConversions from one SI prefix to another (within 1 of the 7 base units) can easily be preformed by moving the decimal place of a quantity by 1 space or 3, left or right.
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Practice Problems
1. 5.6 cm to m
2. 56 mg to g
3. 340 mm to cm
4. 1.2 ML to L
0.056 m0.056 g34 cm
1,200,000 L
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Using SI prefixes: Factor-Label Method (Dimensional Analysis)
Method requires translating two equal quantities into a ratio or conversion factor Ex: 16 oz = 1 lb can be written 16 oz or
1 lb 1 lb 16 oz
Notice: a conversion factor can be represented 2 ways!
This can be done with any 2 equal quantities 2 grand slams = 8 R.B.I.’s 1 fortnight = 14 days 100 cm = 1 m
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Using SI prefixes: Factor Label Method
Using the factor label method to solve problems
Ex: How many dimes are in 14 dollars?1. Write the given2. Write conversion factor3. Solve, crossing out units that have
divided out
14 dollars x 10 dimes = 14o dimes
1 dollar
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Using Factor-Label Method
Sample Problems:
Converting 9.8 g to kg
9.8 g x 1 kg = 0.0098 kg 1000. g
Converting 9.8 kg to g
9.8 kg x 1000. g = 9800 g 1 kg
“1” goes in front of larger unit!
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Practice Problems
Try these practice problems, but now using the Factor-Label Method (I realize this seems like more work than the
number line method…but there’s a reason why we have to learn this)
1. 5.6 cm to m
2. 1.2 L to ML
3. 100 mm to cm
4. 25 kg of water to mL
0.056 m1.2 x 10-6
ML
10 cm
2500 mL
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Density Practice
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Taking Measurements in ChemistryAccuracy vs. Precision
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Accuracy & Precision in Measurements
Accuracy: closeness of measurements to correct value
Precision: closeness of a set of measurements to each other (assuming they’re made in the same way)
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High accuracyHigh precision
Low accuracyHigh precision
Low accuracyLow precision
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Accuracy vs. Precision
Example: A student measures the density of a sample of nickel.
The density of nickel is 8.9 g.mL -1
So the results were: Precise, but not accurate
Density Result (g.mL -1)
Trial 1 7.8
Trial 2 7.7
Trial 3 7.8
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Accuracy & Precision (continued)
Some error always exists in measurements Skill of measurer Conditions of measurements Limitation of instruments
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Percentage Error
Accuracy of an individual value (or average) can be compared to the correct/accepted value
% Error = Experimental – Accepted x 100
Accepted
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Percentage Error
What is the percentage error for a mass measurement of 17.7 g, given that the correct value is 21.2 g?
A volume is measured experimentally as 4.26 mL. What is the percentage error, given that the accepted value is 4.15 mL?
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Taking Measurements in ChemistrySignificant Figures
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Exploring Uncertainty and PrecisionThe Paper Clip Activity
Measuring always involves some degree of estimation (i.e. uncertainty)
Ruler #3 required the least
amount of estimation because
instrument had greater precision
(more markings)
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Significant Figures
Certain digits: digits that represent a marking on a scale or non-blinking number of a display
Uncertain (estimated) digits: digits that represents the space between the marks on a scale or the blinking number on a display
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Sig Figs: Using the Pacific/Atlantic Rule
Step 1: Ask yourself: is the decimal point present or absent?
Step 2: Determine which way to start counting
If the decimal point is present, start counting from the LEFT
If the decimal point is absent, start counting from the RIGHT
PACIFIC
ATLANTIC
resent bsent
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Pacific/Atlantic Rule
Step 3: Start counting on Pacific or Atlantic side from the first NON-ZERO number. Count all numbers after the first non-zero number including zeros.
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Pacific/Atlantic Rule
Examples:
a) 1234 = ________ sig figs
b) 1204 = ________ sig figs
c) 0.00234 = _______ sig figs
d) 1230 = ______ sig figs
e) 1234.0 = ______ sig figs
44
335
Absent
Absent
Absent
Present
Present
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Pacific/Atlantic Rule
Examples:
a) 1234 = ________ sig figs
b) 1204 = ________ sig figs
c) 0.00234 = _______ sig figs
d) 1230 = ______ sig figs
e) 1234.0 = ______ sig figs
3 certain digits – indicated by lines on measuring device ; 1 estimated digit - in between lines3 certain ; 1
estimated
2 certain ; 1 estimated
(zero is a place holder)
2 certain ; 1 estimated
(zero’s are place holders)
4 certain ; 1 estimated
5
3
344
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Rounding Sig. Figs.
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Using Sig. Figs. In Calculations
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Do Now: Precision of Lab Instruments
1. Record the following quantities to the correct number of decimal places.
________ L ________ mL _______ oC
2. Convert your answer in A to milliliters: ________ mL
3. Add your answer from A & B. Record using correct sig. figs. ________ mL
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Scientific Notation
Some numbers are very large or very small, so we need a short hand notation.
602,200,000,000,000,000,000,000
6.022 x 1023
0.0000000000000000000000199
1.99 x 10-23
Too large:
Too small:
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Scientific NotationN x 10n
N is a number between 1 and 10
n is a positive or negative integer
if n is a negative number, the full number is a small decimal
if n is a positive number, the full number is a large number
3.69 x 10-4 ________________1.245 x 105 ________________