PP 4 units of measurement and precision vs accuracy 1 3 4

 We also make QUANTITATIVE We also make QUANTITATIVE observations that involve MEASUREMENTS with numbers and units units...  In Chemistry we use SI Units of measureInternational S

Units of Measurement Precision vs Accuracy Sections 1.3 and 1.4

Types of Observations and Measurements

 We make QUALITATIVE We make QUALITATIVE observations of

reactions — changes in color and physical

state.

 We also make QUANTITATIVE We also make QUANTITATIVE

observations that involve

MEASUREMENTS with numbers and

units

units

A measurement always has two parts:

 In Chemistry we use SI Units of measure

International System of Units

 These are the base units that we will be

working with:

 From these base units, we can combine them using mathematical operations to obtain derived units

 Some of the derived units we will be using include:

 Density (g/ml)

 Area (m2)

 Volume (m3)

Prefixes Used in the SI System

.

To convert from one metric unit to another – remember this

pneumonic: King Henry Died (Unexpectedly) Drinking Chocolate Milk

You must also know…

…how to convert within the Metric

System Here’s a good device:

On your paper draw a line and add 7 tick marks:

Above the tick marks write the

abbreviations for the King Henry pneumonic:

k h d (u ) d c m

Write the units in the middle under the “U”.

m l g

Let’s add the labels for meter, liters, and grams:

How to use this device:

1 Look at the problem Look at the unit

that has a number On the device put

your pencil on that unit.

2 Move to new unit, counting jumps and noticing the direction

of the jump

3 Move decimal in original number the same # of spaces and

in the same direction

Example #1:

(1) Look at the problem: 56 cm = _ mm

Look at the unit that has a number 56 cm

On the device put your pencil on that unit.

k h d u d c m

km hm dam m dm cm mm

Example #1:

k h d u d c m

km hm dam m dm cm mm

2 Move to new unit, counting jumps and noticing the direction of

the jump! Be careful NOT to count the spot you start from, where you put your pencil point Only count the jumps!

One jump to the right!

Example #1:

3 Move decimal in original number the same # of spaces and in

the same direction

56 cm = _ mm

Move decimal one jump to the right 56.0

Add a zero as a placeholder.

Example #2:

(3) Move decimal in original number

the same # of spaces and in the same direction.

7.25 L = kL

Move decimal to the left three jumps.

Add two zeros as placeholders.

Answer: 7.25 L = 00725 kL

Three jumps

to the left!

Mass and Weight

• Mass and weight are directly related as long as we

remain on earth at the same elevation That is, if one object has twice the mass of another, then its weight on earth would also be twice as large.

• Weight is force of the

gravitational pull on an

object (pounds, measured with a scale) It would be different on the moon than it

What is a Liter??

Measuring Volume:  The volume of a liquid is

measured with a

graduated cylinder

When liquid is poured

into the cylinder, a curved

surface called the

meniscus is formed.

1 1000 m = 1 _ a) mm b) km c) dm

2 0.001 g = 1 _ a) mg b) kg c) dg

3 0.1 L = 1 _ a) mL b) cL c) dL

4 0.01 m = 1 _ a) mm b) cm c) dm

Measured Numbers

When you use a measuring tool is used

to determine a quantity such as your height or weight, the numbers you

obtain are called measured numbers.

 Not obtained with measuring tools

limit the number of significant figures

reported.

D. _There were 6 hats on the shelf

E. _A can of soda contains 355 mL of soda

Solution

Classify each of the following as an exact (e)

or a measured(m) number Give reason.

A m Requires a thermometer(measuring tool)

B e From a definition in U.S system

C m Need measuring tool to determine

D e Counted the hats

E m Measured

 Two types of instruments used to take measurements.

electronically by machine (Ex: electronic balance)

numbered lines to determine measurement (Ex: ruler)

UNCERTAINTY IN YOUR MEASUREMENT.

 A digit that must be estimated is called

uncertain

degree of uncertainty.

 Record the certain digits and the first

uncertain digit (the estimated number).

 Graduated cylinders shows each line (scale) represents 1

ml increments

ones place, therefore

tenth place

estimated digit must be

determined by YOU.

43 ml The zero, in tenth

place, is the estimated digit.

Measurement of Volume Using a Buret  The volume is read at the bottom of the liquid curve

 Electronic thermometer is an

example of a instrument that uses digital display.

 The last digit in these types of

instruments is the estimated digit and is always supplied

 The 6 in the tenth place in the

estimated digit.

 32.6 oC is the correct reading.

 All measurements have some degree of uncertainty WHY????? All measurements have some degree of uncertainty WHY?????

 When measurements are recorded CORRECTLY it must be

written with a

written with a digit of uncertainty (estimated digit) digit of uncertainty (estimated digit) and a unit. and a unit .

 Last digit in ANY measurement is the digit of uncertainty Last digit in ANY measurement is the digit of uncertainty

(estimated digit).

Precision and Accuracy

Accuracy

Precision

of the same quantity

 Accurate means "capable of providing a

correct reading or measurement."

 In Chemistry: it means 'correct‘ How

close you are to the accepted value.

Accurate

Precise X

XX

X XX

Can you be neither accurate or precise?

This is a random like pattern, neither precise nor accurate The darts are not clustered together and are not near the bull's eye

x

x

x

xx

This is an accurate pattern, but not precise The darts are not clustered, but their 'average' position is the center

of the bull's eye

This pattern is both precise and accurate The darts are tightly clustered and their average position is the center

of the bull's eye

xxx

x x