Chemistry

Chapter 3

Scientific Measurement

Qualitative Qualitative MeasurementMeasurement

•Gives results in a descriptive form

•Nonnumeric

Quantitative Quantitative MeasurementMeasurement

►Gives results in a Gives results in a definite formdefinite form

►Usually as numbers Usually as numbers and unitsand units

Scientific Notation

•Shorthand way to express very large and very small numbers

Example3.6 x 104

= 3.6 x 10 x 10 x 10 x 10

= 36 000

0.0081 =8.1 x 10-3

Direction of decimal movement

To the left is +

To the right is -

–Operations with numbers in scientific notation

MultiplicationMultiplication

–Multiply the numbers and then add the exponents

DivisionDivision

–Divide the numbers and subtract the exponents

Addition and subtraction

Exponent must be the same to proceed

Must move the decimal appropriately and then adjust the exponent

Then you can solve the problem

Measured values only as reliable as the instrument used to take the measurement!

Uncertainty in measurement

AccuracyMeasure of how close a measurement comes to the actual or true value

PrecisionPrecision–Measure of how close a Measure of how close a series of measurements series of measurements are to one anotherare to one another

Pg. 64 Dartboard example

In class: Pg. 97 #80

Evaluating the accuracy of a Evaluating the accuracy of a measurementmeasurement

• Percent error

• Percent error = [error] X 100

accepted value

Error - the difference between the accepted value and the experimental value (absolute value)

• Experimental value – measured in the lab

• Accepted value – correct value based on reliable references

• Pg. 65 example

Everyone understand so far?

Good!!!

Significant figures in measurements (sig figs)

Rules page 66-67

Sample problems

Pg. 68

Sig Figs in Calculations

Rules for rounding Pg. 68Page 69 Sample

Solving problems with sig figs

Multiplying and dividing with sig figs

The answer you get must be rounded to the same number of sig figs as the measurement with the lowest number of sig figs (that you multiplied or divided)

Example

Multiply 4.610 feet by 1.7 feet. Express your answer in correct sig figs

4.610 x 1.7 = 7.837 How do you round it? 4.610 has 4 sig figs 1.7 has 2 sig figs Round answer to 2 sig figs Answer = 7.8 square feet

Adding and Subtracting with sig figs

When adding or subtracting measurements, the answer cannot have more certainly than the least certain measurement.

Answer must have the same number of sig figs to the right of the decimal point as the measurement with the fewest sig figs to the right of the decimal point

Example

4.271 grams (3 sig figs to the right of decimal)

2 grams (0 sig figs to the right of decimal)

+ 10.0 grams (1 sig fig to the right of decimal)

16.271 grams round 16 grams

Handout practice – work with a partner!

Grab a calculator

SI System of UnitsSI System of Units

•Page 73 Units of measurement

•Table 3.1

• Metric system established in France in 1790

• SI Adopted by international agreement in 1960

PrefixesPage 74 Table 3.2

Length

SI unit - meter (m)

Pg. 74 Table 3.3

VolumeSpace occupied by any sample of matter

L x W x H“Derived” unitPg. 75 Table 3.4

•Volume of a cube 1m on each side

•SI unit = m3 •More common to use Liter (L) = dm3

1 Literthe volume occupied by a cube 10 cm on each side

10 cm x 10 cm x 10 cm = 1000 cm3

1000 cm3 = 1 L

1 dm = 10 cm

1 L = 1 dm3

1 mL = 0.001 L1000 mL = 1 L1000 cm3 = 1000 mL = 1 L

•Volumes for solids, liquids, gases change with change in temperature

Much more dramatic with gases

Measuring devices calibrated at 20oC

Room temperature

MassMass•Difference between mass and weight

•SI unit = Kilogram (kg)•1 g = 0.001 kg•1000 g = 1 kg•Pg. 76 Table 3.5

• Will show on board something special about H2O

Temperature Scales

CelsiusKelvin

Absolute zero

Kelvin scale explanation

Heat measurement

calorie Joule

1 cal = 4.184 J 1J = 0.2390 cal

Unit Conversions

Also called “factor labeling”How many inches in 2 feet?How many feet in 36 inches?You just did a unit

conversion!!!!!!Look at board

Must use correct “conversion factor”

• 230 cm = ? m• Must know that 100 cm = 1m• Write possible conversion

factors• 1m or 100 cm

100 cm 1 m

Write the number you are converting first

Multiply it by the conversion factor that has the unit you want your answer to be in on the TOP

This guarantees that you will divide or multiply when you are supposed to.

• 230 cm x 1 m = 2.3 m

100 cm

The top and bottom units cancel out and the only unit left is the one you want you answer to be in!!!!!

Groups!!

Pg. 84-85 # 32-35

Two step conversions

4500 cm = ? km

Derived units

What does “derived” mean?

A derived unit is a measurement unit created by multiplying or dividing other units

Miles per hour words per minute

Area Area Length x width ft x ft = ft2 ft2 is a derived unit (derived from

two length units) m x m = m2 m2 is a derived unit (derived from

two length units)

Volume

Length x width x height

ft x ft x ft = ft3

m x m x m = m3

cm x cm x cm = cm3

Density

Describes how dense something is How heavy it is for its size

Density = mass divided by volume D = M V M = D x V

V = M D

Since you are dividing two different measurements, the unit for density is a DERIVED UNIT.

Derived from a mass measurement and a volume measurement

g/mL g/L

Density problem

Calculate the density of a substance with a mass of 24.3 g and a volume of 32.9 mL. Use the correct unit and the correct number of sig figs in your answer.

D = M V D = 24.3 g 32.9 mL Ans. = 0.739 g/mL

Problem

What is the volume of an object with a density of 1.25 g/mL and a mass of 281 g?

V = M D V = 281 g 1.25 g/mL g cancels, so units are mL for answer V = 225 mL

Volume of irregularly shaped object Water displacement

Go over hw