CHEMISTRY-CPCHAPTER 3

SCIENTIFIC MEASUREMENT

In this chapter, you will apply the scientific method to various problems and use

experiments to prove hypotheses. You will also learn the basic mathematical skills needed to

succeed in chemistry.

is also known as the central science

• Chemists are employed in dozens of occupations

• Whatever your career choice is, chances are you will need some

knowledge of chemistry!!!!

The Scientific Method

Hypothesis: A Testable Prediction

• If…then… statement

• Narrow—tests one, and only one, thing

Example 1: The static on your radio increases right before it thunders during a storm.

Example 2: People who smoke cough more than people who don’t smoke.

Hypothesis: A Testable Prediction

• If…then… statement

• Narrow—tests one, and only one, thing

Example 3: You sneeze every time you visit your best friend’s house.

Example 4: On a cold morning, the air pressure in the tires of your car measures 34 psi. After several hours of high-speed driving, the pressure measures 38 psi.

EXPERIMENT

Variable: The factor being tested in an experiment

• Independent Variable: The factor that you change/adjust in the experiment

• Dependent Variable: The factor that changes due to changes in the independent variable.

EXPERIMENT

Control: Factor that responds in a predictable way to the experiment

– A control is what the rest of the experiment can be compared to

Constant: Factor(s) that do

not change during the

experiment.

Example: What experiment could be done to prove/disprove the following hypothesis?

“Clean” laundry detergent causes skin rash.

• Independent Variable:• Dependent Variable:• Control:• Constant:

EXPERIMENT

• Data: Recorded Observations– Qualitative:

– Quantitative:

• Graph: a visual representation of data

Graph: a visual Graph: a visual representation of datarepresentation of data

x-axis: the horizontal axisx-axis: the horizontal axis Independent Variable: The factor in the Independent Variable: The factor in the

experiment that the experimenter experiment that the experimenter changes.changes.

y-axis: the vertical axisy-axis: the vertical axis Dependent Variable: The factor that Dependent Variable: The factor that

changes due to changes in the changes due to changes in the independent variable.independent variable.

TABLETABLE

Independent Variable Dependent Independent Variable Dependent VariableVariable

Should have at

least 3 pieces of

data!

Y-a

xis

x-axis

Steps to GraphingSteps to Graphing

Numbering: Make sure the numbers Numbering: Make sure the numbers you put on the axes follow patterns.you put on the axes follow patterns. For example: 2, 4, 6, 8, 10 or 5, 10, 15, For example: 2, 4, 6, 8, 10 or 5, 10, 15,

20 or 0.1, 0.2, 0.3, 0.4 etc.20 or 0.1, 0.2, 0.3, 0.4 etc. Labeling: Make sure you label each Labeling: Make sure you label each

axis with a title and a unit and that axis with a title and a unit and that you title your graph.you title your graph.

TrendsTrends

Best Fit Line: A straight line that Best Fit Line: A straight line that goes through the center of most goes through the center of most points.points.

Trends cont.Trends cont.

Inversely Proportional: As one Inversely Proportional: As one variable increases, the other variable increases, the other variable decreases.variable decreases.

Trends in GraphingTrends in Graphing

Directly Proportional: As one Directly Proportional: As one variable increases/decreases the variable increases/decreases the other does the sameother does the same

Y-a

xis

x-axis

Example: Create a line graph of the following data: Mass (g)Mass (g) Volume Volume (cm(cm33))

2525 100100

3030 115115

4040 134134

5050 160160

5454 163163

Draw Conclusions

Theory: Explains

• States the “Why”

Law: States a Fact

• States the “What”

Uncertainty in Measurements

Why are measurements uncertain? Precision of instrumentation varies Human error

Reading Measurements The number of digits you should write

when writing down a measurement depends on the instrumentation you are using.

You should always include a number and a unit when writing down a measurement

When determining a measurement include all the digits you know for certain plus 1 more digit.

Graduated Cylinder

Put the cylinder flat on the table and read at the bottom of the miniscus (bubble)

Ruler

Thermometer

Significant Figures

All the digits you know for certain in a measurement + 1 more digit

What is the difference between precision & accuracy?

Precision Also called reproducibility or repeatibility Measurements are close to each other (getting

the same measurements each time)

Accuracy

Measurements are close to the actual value

PERCENT ERROR

Percent Error: |measured value – accepted value| x 100%

accepted value

You measure the classroom temperature to be 23C. The actual classroom temperature is 20 C. What is your percent error?

ROUNDING

If the next number is 5 or greater, round the last number up 1. If not, do nothing.

Scientific Notation

A number is written in 2 parts. The first part is a number between 1 & 10 The second part is a power of ten

Exponent Positive exponents represent numbers

greater than 1 Negative exponents represent numbers less

than 1

Scientific Notation To convert a number to scientific notation:

Count how many places the decimal place must be moved to make the number a number between 1 & 10 (the coefficient) The number of spaces the decimal moved is the value of the

exponent If you moved the decimal to the right, the exponent is negative If you moved the decimal to the left, the exponent is positive Write: Coefficient x 10exponent

To convert a number from scientific notation to regular notation: If the exponent is positive, move the decimal in the coefficient

the number of spaces indicated by the exponent to the right If the exponent is negative, move the decimal in the coefficient

the number of spaces indicated by the exponent to the left.

Scientific Notation Example 1: Express each of the following in

scientific notation.8960 = 36,000,000 =

0.00023 = 0.000 000 025 3 =

Example 2: Express each of the following numbers in regular notation.4.563 x 107 = 2.53 x 10-3 =

6.805 x 108 = 1.33450 x 10-7 =

Scientific Notation

A number is written in 2 parts. The first part is a number between 1 & 10 The second part is a power of ten

Exponent Positive exponents represent numbers

greater than 1 Negative exponents represent numbers less

than 1

Calculating in Scientific Notation(Do not change the numbers out of scientific notation when calculating)

Without Calculator

With Calculator

(5.5 x 106) x (1.111 x 10-1) =

(9.896 x 10-34) (3.311 x 10-24) =

Significant Figures

All the digits you know for certain in a measurement + 1 more digit

Significant figures reveal the precision of a measurement and the measuring device

SIGNIFICANT FIGURESTo count significant figures, if there is a

decimal, count all digits including and after the first non-zero

number. If there is not a decimal, start counting at the first non-

zero number but do not count zeroes at the end of the number.

123 = _____ sig figs0.0025 = _____ sig figs5007= ______ sig figs470 = ___ sig figs470.0 = ___ sig figs 0.00470 = ____ sig figs2.020 x 104 = ____ sig figs

ROUNDING The first significant digit is the first nonzero number.

In a number without a decimal, zeroes at the end are not significant and therefore can be added as placeholders

Count the appropriate # of sig figs, if the next number is 5 or greater, round the last number up 1. If not, do nothing. Examples:

2.3344(1)

1.029 (3)

0.00234(2)

5060 (2)

213488 (2)

SIGNIFICANT FIGURES IN CALCULATIONSMultiplication/Division: The measurement with the

smallest number of significant figures determines how many

significant figures are allowed in the final answer.

Addition/Subtraction: The measurement with the smallest

number of decimal places determines how many decimal

places are allowed in the answer.

SIGNIFICANT FIGURES IN CALCULATIONS

0.3287 g x 45.2 g =

125.5. kg + 52.68 kg + 2.1 kg =

0.258 mL 0.36105 mL =

68.32 ns – 1.001 ns – 0.00367 ns =

What do the countries in red have in common?

International System of Units (SI Units) A revised version of the metric system that

was developed in France in 1795 and was adopted by international agreement in 1960

There are 7 base SI units All other SI Units are DERIVED from the 7 base

units

Base Units: The 7 metric units that SI is built upon

Physical Quantity

Unit Name Unit Symbol Measured using…

Mass Kilogram kg Balance

Length Meter m Meterstick/Ruler

Time Second s Stopwatch

Quantity Mole mol varies

Temperature Kelvin K thermometer

Electric Current Ampere A Ammeter

Luminous Intensity

Candela cd Photometer

NON-SI UNITS

Physical Quantity Unit Name Unit Symbol

Volume Liter L

Pressure Pascal

Atmosphere

Pa

Atm

Temperature Celsius C

Energy Joule J

Derived Units Commonly Used in Chemistry

Physical Quantity How To Calculate: Unit Symbol

Volume Length x Width x Height

Area Length x Width

Density Mass Volume

To Derive a Unit• Write the mathematical formula for the quantity.• Replace the formula with units and simplify.

Density

Density = Mass Volume

a) Calculate the density of a piece of a cube of iron with a mass of 1.23 kg that measures 2 cm on each side.

Temperature

• Measured with: Fahrenheit Scale: An arbitrary scale created by Gabriel Fahrenheit.

F = (C 9/5) + 32

Celsius Scale: Based on the freezing and boiling points of water.

C = (F – 32) 5/9

C = K – 273

Kelvin Scale: The S.I. Scale

•Based on absolute zero.

Absolute Zero: The point at which the motion of particles of matter (their kinetic energy) ceases.

K = C + 273

METRIC CONVERSIONS

METRIC PREFIXES

METRIC PREFIXESPREFIX In 1 base unit there

are:Example

mega- (M) 10-6 M-unit 1 m = 10-6 Mm

kilo- (k) 10-3 k-unit 1 L = 10-3 kL

deka- (dk) 0.1 dk-unit 1 g = 0.1 dkg

BASE UNIT

deci- (d) 10 d-unit 1 s = 10 ds

centi- (c) 100 c-unit 1 mol = 100 cmol

milli- (m) 1000 m-unit 1 m = 1000 mm

micro- () 106 -unit 1 L = 106 L

nano- (n) 109 n-unit 1 g = 109 ng

pico- (p) 1012 p-unit 1 s = 1012 ps

DIMENSIONAL ANALYSIS

What is dimensional analysis?

What is a unit equality?

What is a conversion factor?

What is a conversion factor equal to?

How do you use conversion factors?

Dimensional analysis is a method used to convert between units

A unit equality are two values that are equal to each other (like 3 ft = 1 yd)

A conversion factor is a ratio of a unit equality (3 ft/1yd)

1 (anything divided by an equal value is 1)

Multiply it to a number to convert between units

Steps to Dimensional Analysis1. Start with what you know

(number and unit).2. Times a line.3. Add a conversion factor so that

units cancel and what you are looking for is on top of the ratio.

4. Check your answer.

DIMENSIONAL ANALYSIS

1 Base Unit Equals

10-6 Mega-10-3 kilo-0.1 deka-10 deci-

100 centi-1000 milli-106 micro-109 nano-1012 pico-