Transcript Chem Comm 2. Measurements

Unit 2.
Measurement
Do Now
 In
your own words, what do you think is the
difference between:

Accuracy and Precision?
A. Accuracy vs. Precision
 Accuracy
- how close a measurement is to the
accepted value
 Precision
- how close a series of measurements are
to each other
ACCURATE = CORRECT
PRECISE = CONSISTENT
ACCURATE = CORRECT
PRECISE = CONSISTENT
B.
Percent Error
 The
accuracy of an individual value can be
compared with the correct or accepted
value by calculating the percent error.
 Percent
error is calculated by subtracting
the accepted value from the experimental
value, dividing the difference by the
accepted value, and then multiplying by
100.
B. Percent Error

Indicates accuracy of a measurement
% error 
experimental  literature
literature
your value
accepted value
 100
B. Percent Error

A student determines the density of a substance to be
1.40 g/mL. Find the % error if the accepted value of
the density is 1.36 g/mL.
% error 
1.40 g/mL  1.36 g/mL
1.36 g/mL
% error = 2.90 %
 100
Objectives:






Describe the difference between a
qualitative and a quantitative measurement.
Describe the difference between accuracy
and precision.
Write a number in scientific notation.
State the appropriate units for measuring
length, volume, mass, density, temperature
and time in the metric system..
Calculate the percent error in a
measurement.
Calculate density given the mass and
volume, the mass given the density and
volume, and the volume given the density
and mass.
Chapter
2
Section 1
Scientific
Method
 Scientific
Method is a logical approach to
solving problems by observing and collecting data,
formulating hypotheses, testing hypotheses and
formulating theories that are supported by data.
 Observations
Hypothesis
Theory
Experimentation
Observations
•
Collecting data
•
Measuring
•
Communicating with other scientists
Measurements
 Measurements
are divided into two sets:
– a descriptive measurement.
Color, hardness, shininess, physical state.
(non-numerical)
 Qualitative
– a numerical measurement.
Mass in grams, volume in milliliters, length in
meters.
 Quantitative
Hypothesis
A
tentative explanation that is consistent
with the observations (educated guess).
 An
experiment is then designed to test the
hypothesis.
 Predict
the outcome from the experiments
Theory
 Attempts
to explain why something
happens.
 Has
experimental evidence to support
the hypothesis.
 Observations,
data and facts.
Classwork
 What
is the scientific theory?
 What
is the difference between qualitative and
quantitative measurements?
 Which
of the following are quantitative?
a. The liquid floats on water?
b. The metal is malleable?
c. A liquid has a temperature of 55.6 oC?
 How do hypothesis and theories differ?
Units of
Measurement
 Measurements
represent quantities.
A
quantity is something that has
magnitude, size or amount.
 All
measurements are a number plus a
unit (grams, teaspoon, liters).
 Quantity
- number + unit
A. Number vs. Quantity
UNITS MATTER!!
B. SI Units
Quantity
Symbol
Base Unit
Abbrev.
Length
l
meter
m
Mass
m
kilogram
kg
Time
t
second
s
Temp
T
kelvin
K
Amount
n
mole
mol
B. SI Units
Prefix
mega-
Symbol
M
Factor
106
kilo-
k
103
BASE UNIT
---
100
deci-
d
10-1
centi-
c
10-2
milli-
m
10-3
micro-

10-6
nano-
n
10-9
pico-
p
10-12
SI Prefix Conversions
1) 20 cm = ______________ m
2) 0.032 L = ______________ mL
3) 45 m =
______________ m
Derived SI Units
 Many
SI units are combinations of the
quantities shown earlier.
 Combinations
of SI units form derived
units.
 Derived
units are produced by
multiplying or dividing standard units.
C. Derived Units cont…
 Combination
 Volume

of base units.
(m3 or cm3)
length  length  length
1 cm3 = 1 mL
1 dm3 = 1 L
 Density
 (kg/m3 or g/mL or g/cm3)
mass per volume
M
D=
V
Volume
(m3) is the amount of space occupied
by an object.
 Volume
 length
 Also
x width x height
expressed as cubic centimeter (cm3).
 When
measuring volumes in the laboratory a
chemist typically uses milliliters (mL).
 1 mL =1 dm3 = 1 cm3
Density
 Density
is a characteristic physical
property of a substance.
 It
does not depend on the size of the
sample.
 As
the sample’s mass increases, its
volume increases proportionally.
 The
ratio of mass to volume is constant.
Density…
 Calculating
 You
density is pretty straight forward
need the measure Mass and Volume
 Mass-
Obtain by weighing the mass of an object by
using a balance and then determine the volume.
Volume
 Solids
- the volume can be a little difficult.
 If
the object is a regular solid, like a cube, you
can measure its three dimensions and
calculate the volume.
 Volume
= length x width x height
Volume Cont….
If the object is an irregular solid, like a rock, determining
the volume is more difficult.
Archimedes’ Principle – states that the volume of a solid is
equal to the volume of water it displaces.
Put some water in a graduated cylinder and read the
volume. Next, put the object in the graduated cylinder and
read the volume again.
The difference in volume of the graduated cylinder is the
volume of the object.
Volume Displacement
A solid displaces a matching volume of
water when the solid is placed in water.
35mL
25 mL
Learning Check
What is the density (g/cm3) of 48 g of a metal if the
metal raises the level of water in a graduated
cylinder from 25 mL to 33 mL?
1) 0.2 g/cm3
2) 6 g/cm3
3) 252 g/cm3
PROBLEM: Mercury (Hg) has a density of
13.6 g/cm3. What is the mass of 95 mL of
Hg in grams?
PROBLEM: Mercury (Hg) has a density of 13.6
g/cm3. What is the mass of 95 mL of Hg?
 First,
note that
1
3
cm
= 1 mL
Strategy
Use density to calc. mass (g) from volume.
 Density
=
Mass
Volume
PROBLEM: Mercury (Hg) has a density of 13.6
g/cm3. What is the mass of 95 mL of Hg?
 Density
=
Mass
Volume
13.6g/cm
3
Mass (g)
=
95 mL
Learning Check
Osmium is a very dense metal. What is its
density in g/cm3 if 50.00 g of the metal
occupies a volume of 2.22cm3?
1) 2.25 g/cm3
2) 22.5 g/cm3
3) 111 g/cm3
Solution
Placing the mass and volume of the osmium metal
into the density setup, we obtain
D = mass =
volume
50.00 g =
2.22 cm3
= 22.522522 g/cm3 = 22.5 g/cm3
Learning Check
The density of octane, a component of
gasoline, is 0.702 g/mL. What is the mass,
in kg, of 875 mL of octane?
1) 0.614 kg
2) 614 kg
3) 1.25 kg
Learning Check
The density of octane, a component of
gasoline, is 0.702 g/mL. What is the mass,
in kg, of 875 mL of octane?
1) 0.614 kg
D. Density
Mass (g)
Δy M
D

slope 
Δx V
Volume (cm3)
Problem-Solving Steps
1. Analyze
2. Plan
3. Compute
4. Evaluate
D. Density
 An
object has a volume of 825 cm3 and a density of 13.6
g/cm3. Find its mass.
GIVEN:
V = 825 cm3
D = 13.6 g/cm3
M=?
M
D
V
WORK:
D. Density
 An
object has a volume of 825 cm3 and a density of 13.6
g/cm3. Find its mass.
GIVEN:
WORK:
V = 825 cm3
D = 13.6 g/cm3
M=?
M = DV
M
D
V
M = (13.6
g/cm3)(825cm3)
M = 11,200 g
A
D. Density
liquid has a density of 0.87 g/mL. What volume is
occupied by 25 g of the liquid?
GIVEN:
D = 0.87 g/mL
V=?
M = 25 g
M
D
V
WORK:
A
D. Density
liquid has a density of 0.87 g/mL. What volume is
occupied by 25 g of the liquid?
GIVEN:
WORK:
D = 0.87 g/mL
V=?
M = 25 g
V=M
D
M
D
V
V=
25 g
0.87 g/mL
V = 29 mL
III. Unit Conversions
A. SI Prefix Conversions
1.
Find the difference between the exponents of the
two prefixes.
2.
Move the decimal that many places.
To the left
or right?
A. SI Prefix Conversions
move right
move left
Prefix
mega-
Symbol
M
Factor
106
kilo-
k
103
BASE UNIT
---
100
deci-
d
10-1
centi-
c
10-2
milli-
m
10-3
micro-

10-6
nano-
n
10-9
pico-
p
10-12
A. SI Prefix Conversions
1) 20 cm =
______________ m
2) 0.032 L = _____________ mL
3) 45 m =
______________ nm
4) 805 dm = ______________ km
A. SI Prefix Conversions
0.2
1) 20 cm = ______________
m
32
2) 0.032 L = ______________
mL
3) 45 m =
45,000
______________
nm
0.0805
4) 805 dm = ______________
km
C. Johannesson
Conversion Factors
Conversion Factors
factor – a ratio derived from the
equality between two different units that can
be used to convert from one unit to the other.
 Conversion
 Example:
the conversion between quarters
and dollars:
4

quarters
1 dollar
or
1 dollar
4 quarters
Conversion Factors
Conversion Factors
 Example:
 Determine
 Number
?
the number of quarters in 12 dollars?
of quarters = 12 dollars x conversion factor
Quarters = 12 dollars x 4 quarters = 48 quarters
1 dollar
Learning Check
Write conversion factors that relate
each of the following pairs of units:
1. Liters and mL
2. Hours and minutes
3. Meters and kilometers
B. Dimensional Analysis
 The

“Factor-Label” Method
Units, or “labels” are canceled, or “factored” out to
make your calculations easiers
g
cm 

g
3
cm
3
B. Dimensional Analysis
 Steps:
1. Identify starting & ending units.
2. Line up conversion factors so units cancel.
3. Multiply all top numbers & divide by each
bottom number.
4. Check units & answer.
Learning Check
1) A
rattlesnake is 2.44 m long. How
long is the snake in cm?
a) 2440 cm
b) 244 cm
c) 24.4 cm
2.44 m x 100 cm
1m
= 244 cm (b)
B. Dimensional Analysis
2) Your European hairdresser wants to cut your hair 8.0
cm shorter. How many inches will he be cutting off?
cm
in
8.0 cm
1 in
2.54 cm
= 3.2 in
B. Dimensional Analysis
3) A piece of wire is 1.3 m long. How many 1.5-cm
pieces can be cut from this wire?
cm
pieces
1.3 m 100 cm
1m
1 piece
1.5 cm
= 86 pieces
% Error