welcome to chemistry! with mrs. strain rm. 403

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

Taking Measurements in ChemistryAccording to the Scientific Method

The Scientific Method

Scientific Method: logical approach to solving problems by…a. Observing & collecting datab. Formulating hypothesesc. Testing hypothesesd. Formulating theoriese. Publishing results

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.

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!

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”

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

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:

Taking Measurements in ChemistryGraphing Measurements

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

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

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

Taking Measurements in ChemistryCh. 2 The SI or Metric System

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.

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

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??

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

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

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

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.

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

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

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

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!

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

Density Practice

Taking Measurements in ChemistryAccuracy vs. Precision

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)

High accuracyHigh precision

Low accuracyHigh precision

Low accuracyLow precision

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

Accuracy & Precision (continued)

Some error always exists in measurements Skill of measurer Conditions of measurements Limitation of instruments

Percentage Error

Accuracy of an individual value (or average) can be compared to the correct/accepted value

% Error = Experimental – Accepted x 100

Accepted

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?

Taking Measurements in ChemistrySignificant Figures

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)

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

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

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.

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

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

Rounding Sig. Figs.

Using Sig. Figs. In Calculations

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

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:

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 ________________