Scientific MeasurementChemistry chapter 2

Scientific Measurement

Distinguish between quantitative and qualitative measurements. List SI units of measurement and common SI prefixes. Distinguish between the mass and weight of an object. Convert measurement to scientific notation. Distinguish among the accuracy, precision, and error of

measurement. Identify the number of significant figures in a measurement and in the

result of calculation. Identify and calculate derived units. Calculate the density of an object from experiment data.

TEKS: 2A, 2B, 2C, 2D, 2E, 3C, 3D, 3E, 4B, 4C

Branches of chemistry

organic chemistry—the study of carbon-containing compounds

  inorganic chemistry—the study of non-organic substances   physical chemistry—the study of properties of matter,

changes that occur in matter, and the relationships between matter and energy

  analytical chemistry—the identification of the composition of materials

  biochemistry—the study of the chemistry of living things   theoretical chemistry—the use of mathematics and

computers to design and predict the properties of new compounds

Quantitative vs. QualitativeObservations

Qualitative – observations made with adjectives

“The water is clear and cool.”

Quantitative – observations that include a measurement or other numeric data

“There are 40mL of water.”

Scientific Method

Two parts of measurements

1. Quantity – indicates size or magnitude (how much?)

2. Unit – tells us what is to be measured and compares it to a previously defined size (of what?)

Measurements must have both a quantity and a unit to be valid.

International System of Units

Length – meter Mass – kilogram Temperature – Kelvin Energy – joule Amount of a substance – mole Electric current - ampere Volume – m3

Density – g/cm3

Weight - Newton

Commonly Used Prefixes in the Metric System

Prefix Meaning Exponent

mega (M) 1 000 000 106

kilo (k) 1000 103

hecto (h) 100 102

deka (da) 10 101

deci (d) 1/10 10-1

centi (c) 1/100 10-2

milli (m) 1/1000 10-3

micro (µ) 1/1 000 000 10-6

nano (n) 1/1 000 000 000 10-9

pico (p) 1/1 000 000 000 000 10-12

Conversion Factors

Conversion factors are equalities written in ratio form:

1 km = 1000m 1km = 1000 m

1000 m 1 km

Choose the format that allows you to cancel the

original units and leave the new units.

Ex. 2.5 km = ________ m

You would choose 1000 m km

Foldable

Can use this to simply move your decimal in order to convert between units

Ex. 1 mg = ? g

Conversion Factors

Make sure that you have a valid equality before writing your conversion factor.

Which of these equalities are correct?

1 m = 1 x 10-6 µm1 m = 1 x 106 µm1 x 10-6 m = 1µm

Important Equalities

1 dm

10 cm

1 dm10 cm

1 dm

10 cm

1 dm3 = 1000cm3

1mL = 1cm3 = 1cc

1dm3 = 1000 mL = 1L

100 dm3 = ‗‗‗‗nm3

Conversion Practice

Problem

List in order – largest to smallest

a. 1 dm3

b. 1 µL

c. 1 mL

d. 1 L

e. 1 cL

f. 1 dL

Largest to smallest

A. dm3

D. 1 L F. dL E. cL C. mL B. μL

Derived Units

Derived units are formed from a combination of other units.

Examples include:

m/s & km/hr (speed), cm3 & dm3(volume), J/g·°C (specific heat), g/mol (molar mass), g/cm3 & kg/m3 (density)

Density

Density is the ratio between the mass and volume of an object.

Density = Mass or D = m

Volume V

Density is an intensive physical property.

Density (Math Triangle)

D = M / V

M = D X V

V = M / D

Density Problems

A student finds a shiny piece of metal that she thinks is aluminum. She determined that the metal has a volume of 245 cm3 and a mass of 612 g. Calculate the density. Is the metal aluminum?

The density of silver at 20ºC is 10.5 g/cm3. What is the volume of a 68 g bar of silver?

Density Problems Continued

A weather balloon is inflated to a volume of 2.2 x 103 L with 37.4 g of helium. What is the density of helium, in grams per liter.

A plastic ball with a volume of 19.7 cm3 has a mass of 15.8 g. What is its density? Would the ball sink or float in a container of water?

Specific Gravity

Specific Gravity = Density substance (g/cm3)

Density water (g/cm3)

Making Measurements

Review Scientific Notationrule: move decimal to a number between 1-10

600,000 =

0.00054 =

2.3 X 10-2 =

5.5 X 106 =

Precision and Accuracy

Accuracy refers to the agreement of a particular value with the true value. (how close)

Precision refers to the degree of agreement among several elements of the same quantity. (how repeatable)

Target (a) shows neither accuracy or precision.

Target (b) shows precision, but not accuracy.

Target (c) shows both accuracy and precision.

Uncertainty in Measurement

A digit that must be estimated is called uncertain.

The last digit in a measurement always shows uncertainty.

Significant Digits

Significant Digits show the degree of certainty in a measurement.

Not all digits in a number show certainty, therefore, all digits are not significant.

Counting Significant Digits

Rule 1:

Nonzero integers always count as significant digits.

3456 has 4 “sig digs”

Counting Significant Digits

Rule 2:

Leading zeros do not count as significant figures.

0.0486 has 3 “sig figs”

Counting Significant Digits

Rule 3:

Captive zeros always count as significant figures.

16.07 has 4 “sig digs”

Counting Significant Digits

Rule 4:

Trailing zeros are significant only if the number contains a decimal point.

9.300 has 4 “sig figs”

Counting Significant Digits

Exact numbers have an infinite number of significant figures.

Exact numbers include counting numbers and conversion factors.

Examples: 12 students 1m = 100 cm

Practice Problems

Determine the number of significant figures.a. 12 kilometersb. 0.010 m2

c. 507 thumbtacks d. 0.070020 m

e. 10800 mf. 5.00 m3.g. 2.340 x 103 cmh. 6.02 x 1023 atoms

Rules for Significant Figures in Mathematical Operations

Multiplication and Division: # sig figs in the result equals the number in the least precise measurement used in the calculation.

6.38 cm 2.0 cm = 12.76 cm2

13 (2 sig figs)

Multiplication and Division

Your answer can only have the least number of significant figures in your data.

a.2.0 mL

x 3.00 mL

b. 8432 m = 12.5 m

Rules for Significant Figures in Mathematical Operations

Addition and Subtraction: # sig figs in the result equals the number of decimal places in the least precise measurement.

6.8 cm + 11.934 cm + 3.7556 cm = 22.4896 cm

22.5 cm (1 digit after decimal - 3 sig figs)

Addition and Subtraction

Count the decimal places. You can only have in your answer the least number of decimal places that is seen in your data.

1.0 1 7.00 11.00 + 2.00 - 1.001 + 0.5

+1.000

Rounding Rules

If the digit following the last digit to be retained is:

Then the last digit should:

Example (rounded to 3 sig dig’s)

greater than 5 be increased by 1 38.68 g to 38.7 g

less than 5 stay the same 12.51 m to 12.5 m

5, followed by nonzero digit(s)

be increased by 1 4.8851 cm to 4.89 cm

5, not followed by nonzero digit(s), and preceded by an odd digit

be increased by 1 2.975 kg to 2.98 kg(because 7 is odd)

5, not followed by nonzero digit(s), and the preceding significant digit is even

Stay the same 2.985 kg to 2.98 kg(because 8 is even)

Measurement Tips

Measurement Tools

• Distance

= Meter Sticks & Metric Tapes

• Volume

= Graduated Cylinder

• Time = Stopwatch

• Mass = Balance

• Weight = Spring Scale

Mass vs. Weight

Mass is the amount of matter in an object; weight is the effect of gravity on a mass.

Mass is measured on a balance; weight is measured with a scale.

Mass remains constant at all locations; weight varies with change in gravitational pull.

Volume

1. Never measure in a beaker. They are for estimation only!

2. Place the graduated cylinder on a level surface and read the bottom of the meniscus.

3. Check the scale of the graduated cylinder. Different scales for different sizes!

4. Use displacement to find the volume of irregular solids.

1. Make sure the balance is on a level surface.

Mass

2. Use the same balance in the same place for all parts of a procedure.

3. DO NOT MOVE A BALANCE ONCE IT IS ZEROED!

Length

Rulers & meter sticks wear on the ends – start at a point other than zero.

Choose the unit most reasonable for the item you are measuring – make sure you convert your number accordingly.

Symbols

Δ “Delta” means “change in”

Σ “Sigma” means “sum of”

Graphing Relationships

Direct Relationship-Variables do the same

Straight Line

Inverse (Indirect) Relationship-Variables do the opposite

Parabola

Models

Why do scientists use models in their research???