Transcript 4 Units of Measurement_ Precision vs Accuracy

Units of Measurement
Precision vs Accuracy
Sections 1.3 and 1.4
Types of Observations and
Measurements
 We make QUALITATIVE observations of
reactions — changes in color and physical
state.
 We also make QUANTITATIVE
observations that involve
MEASUREMENTS with numbers and
units.
NO NAKED
NUMBERS
A measurement always has two parts:
A value (this is the number)
A unit of measure (this tells what you
have)
Example:
200 meters; 15 ml; 13.98 grams
 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
.
Metric Prefixes
 Kilo- means 1000 of that unit
 1 kilometer (km) = 1000 meters (m)
 Centi- means 1/100 of that unit
 1 meter (m) = 100 centimeters (cm)
 1 dollar = 100 cents
 Milli- means 1/1000 of that unit
 1 meter (m) = 1000 millimeters (mm)
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:
Next:
Above the tick marks write the abbreviations
for the King Henry pneumonic:
k
h
d
(u )
d
m
l
g
Write the units in the middle under the “U”.
c
m
Let’s add the labels for
meter, liters, and grams:
k
h
d
u
d
c
m
km
hm
dam
m
dm
cm
mm
kl
hl
dal
l
dl
cl
ml
kg
hg
dag
g
dg
cg
mg
Deca can also be dk or da
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.
3.
Move to new unit, counting jumps and noticing the direction of
the jump.
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
km
h
hm
d
u
d
dam
m
dm
c
cm
m
mm
Example #1:
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!
k
km
h
hm
d
dam
u
m
d
c
m
dm
cm
mm
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
56.0.
One jump
to the right!
Move decimal one jump to the right.
Add a zero as a placeholder.
Answer:
56cm = 560 mm
Example #2:
(1) Look at the problem. 7.25 L = ____ kL
Look at the unit that has a number. 7.25 L
On the device put your pencil on that unit.
k
h
d
kl
hl
dal
u
L
d
c
m
dl
cl
ml
Example #2:
2. Move to new unit, counting jumps and
noticing the direction of the jump!
k
h
d
u
kL
hL
daL
L
d
dL
c
m
cL
Three jumps to the left!
mL
Example #2:
(3) Move decimal in original number
the same # of spaces and in the same
direction.
7.25 L = ____ kL
.007.25
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.
Mass and Weight
• Weight is force of the
gravitational pull on an
object (pounds, measured
with a scale). It would be
different on the moon than it
is on earth.
• Mass is a measure of the
amount of matter in an
object. (grams, measured
with a BALANCE)
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.
Learning Check
1. 1000 m = 1
___
a) mm
b) km c) dm
2.
___
a) mg
b) kg
c) dg
___
a) mL
b) cL
c) dL
a) mm
b) cm c) dm
0.001 g = 1
3. 0.1 L = 1
4. 0.01 m = 1 ___
1. 1000 m = 1
___
2.
___
0.001 g = 1
3. 0.1 L = 1
___
4. 0.01 m = 1 ___
a) mm
a) mg
b) km c) dm
b) kg
c) dg
a) mL
b) cL
c) dL
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.
Exact Numbers
 Obtained when you count objects
2 soccer balls
1 watch
4 pizzas
 Obtained from a defined relationship
1 foot = 12 inches
1 meters = 100 cm
 Not obtained with measuring tools
 We will later learn that these numbers do not
limit the number of significant figures reported.
Learning Check
Classify each of the following as an exact(e) or a
measured (m) number.
A.___Gold melts at 1064°C
B.___1 yard = 3 feet
C.___A red blood cell with diameter 6 x 10-4 cm
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.
 Digital display -measurement is displayed
electronically by machine (Ex: electronic
balance)
 Scaled instrument -instruments has
numbered lines to determine measurement
(Ex: ruler)
 REMEMBER TO INCLUDE THE DIGIT OF
UNCERTAINTY IN YOUR MEASUREMENT.

A digit that must be estimated is called
uncertain.
 A measurement always has some 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.
 This instrument is accurate to
ones place, therefore estimated
digit should be in tenth place.
 For scaled instruments the
estimated digit must be
determined by YOU.
 Liquid volume is 43.0 ml not
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 (meniscus).
 Meniscus of the liquid occurs at
about 20.15 mL.
 Certain digits: 20.15
 Uncertain digit: 20.15
 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?????
 When measurements are recorded CORRECTLY it
must be written with a digit of uncertainty
(estimated digit) and a unit.
 Last digit in ANY measurement is the digit of
uncertainty (estimated digit).
Precision and Accuracy
Accuracy
•
Agreement of a particular value with the true value.
Precision
•
Degree of agreement among several measurements
of the same quantity.
Measurement
 Accurate means "capable of providing a correct
reading or measurement."
 In Chemistry: it means 'correct‘. How close you are to
the accepted value.
Measurement
Accurate
Measurement
Precise
X X
X X X
X
Measurement
Can you be neither
accurate or precise?
Measurement
x
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
x
Measurement
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.
x
x
x
x
x
Measurement
This pattern is both
precise and accurate. The
darts are tightly clustered
and their average position
is the center of the bull's
eye.
x
xxx
x