Units Of Measurement

- Mrs. Mawhiney

Measurement of mass, length and volume

- In the United States, we use a fairly awkward

system of measurement for most things - the

English system Scientists use the metric and SI

systems of units for the measurement of physical

quantities - This system using standard units based on very

precisely known properties of matter and light - Prefixes are used in from of the units to

indicate powers of ten

SI Units

Measurement Unit Symbol

Mass Kilogram kg

Length Meter M

Time Second s

Temperature Kelvin K

Quantity Mole mol

Energy Joule J

Pressure Pascal Pa

. Base Units

- Mass - the quantity of matter that a sample

contains - Note that weight is a measure of the attraction

of gravity for a sample and it varies depending

on the distance of the mass to a planet or moon - Scientists often speak imprecisely of the

weight of an amount of substance. They really

mean mass.

Basic SI units/Derived units

- Used to generate new Units
- Volume - space a given quantity of matter

occupies - Volume - expressed in terms of length - m3
- m3 - an inconveniently large volume, so we use

liter (L one cubic decimeter) - We often use a mL (1 cubic centimeter) for more

manageable amounts of matter

Converting between units

- The standard method to convert between two

different units is the factor-label or

dimensional analysis method - Dimensional analysis converts a measurement in

one unit to another by the use of a conversion

factor - Conversion factors are developed from

relationships between units

Measurements and Units

- Measurement - determines the quantity,

dimensions or extent of something - 1.Consist of two parts
- a. a numerical quantity (1.23)
- b. a specific unit (meters)
- Unit - a definite quantity adapted to as a

standard of measurement

Features of Measured Quantities

- When we measure a number, there are physical

constraints to the measurement - Instruments and scientists are not perfect, so

the measurement is not perfect (i. e., it has

error) - The error in the measurement is related to the

accuracy and the precision of the measurement

Accuracy and Precision

- Accuracy how close the measurement is to the

true value (of course we have to know what the

true value is) - Precision is a measure of how closely

individual measurements agree with one another.

Example Accuracy and Precision

Equations for Precision and Accuracy

- 1. Precision
- 2. Accuracy
- Absolute Error
- AE (True value-Avg Value) X 100
- True Value

Counting Significant Figures

- Atlantic / Pacific Method
- a. Absent Decimal- Start on atlantic side

of number cross out all zeroes until 1st

nonzero digit is reached, remaining digits are

significant - b. Present decimal- start on the pacific

side of the number cross out all zeros until

the 1st nonzero digit Is reached, remaining

digits are significant

How many significant figures are in each of the

following measurements?

24 mL

2 significant figures

3001 g

4 significant figures

0.0320 m3

3 significant figures

6.400 x 104 molecules

4 significant figures

560 kg

2 significant figures

Scientific notation and significant figures

- 1. When using scientific notation the base

must be written with the correct number of

significant digits - 2. All zeroes are significant when using

scientific notation