Transcript Scientific Measurement

Chemistry Unit 2:
Scientific Measurement
Salisbury High School
Spring 2007
Chemistry February 12, 2007
1) Begin Unit on
Measurements
2) Notes on measurements,
accuracy, precision
3) HW: WS 2.1
Measurements – Opening
Questions
Questions:
1. What is the purpose of a
measurement?
2. Why are measurements
important to science?
Measurements
Measurements are
fundamental to science
Measurements may be:
a. Qualitative
b. Quantitative
Measurements
Qualitative Measurement:
is a non-numerical
measurement
Example: The solution
turned brown when ammonia
was added to iron (III)
chloride
Quantitative Measurements
Quantitative Measurements:
consist of two parts
a. A number
b. A scale (or unit)
Quantitative Measurements
Scale
(unit)
Example:
The rock has a mass of 9 kg
Number
Practice: Classify as qualitative or
quantitative
1. Water is a liquid
2. The temperature was 9°C.
3. The book is 12 cm long
4. The mixture contains 5 blue
marbles and 12 g of red clay
Terms Used for Measurements
In comparing scientific
results, the following
terms are applied:
a. accuracy
b. precision
Accuracy
Accuracy refers to the
agreement of one
experimental result to the
true or accepted value
The closer the value is to the
true value, the more
accurate one is
Accuracy
Examples of accuracy:
kicking a soccer goal
hitting a three point shot
determining the value of  to be
3.13
Practice: Tell if the accuracy is good or
poor
a. Finding the percent H2O2 to
be 2.9% (the bottle says it
contains 3%)
b. Finding that a 2 Liter bottle
only contains 1.5 Liters of soda
c. Filling a 1000 mL volumetric
flask with 1050 mL of water
Precision
Precision refers to how close
several different experimental
values are to each other
Precision
Examples:
Scoring three goals in a
soccer game
Shooting an air ball five
times
Finding the value for  to be
3.12, 3.15, 3.13
Precision and Accuracy
Problems
Precision problems usually
arise from the skill of the
person doing the experiment
or the division of the
measuring instruments
Precision and Accuracy Problems
Accuracy problems usually
related to the quality of the
equipment used to make
measurements
Precision relates to how
consistent the results are;
accuracy relates to how
correct the results are
Describe the accuracy and precision of
the following
A student makes the
following grades:
a. 99, 100, 98, 100
b. 45, 43, 44, 42
c. 100, 23, 60, 89
Scientific Notation
Occasionally some
measurements are really small or
exceptionally large
1, 400, 000 km, the distance to
the sun
0.000 000 000 066 7, the
Universal Gravitation Constant
To help you use these, you may
express them in scientific notation
Scientific Notation Continued
In scientific notation, a number is
written as the products of two
numbers:
a coefficient and some power of
10
Scientific Notation Continued
The general form for scientific
notation is:
n
M x 10 where
M is  1 but < 10
n is an integer and exponent
{integers must be a positive or
negative whole number}
Rules for Scientific Notation
1. Determine “M” by moving
the decimal point in the
original number to the left or
to the right so that only one
non-zero digit remains to the
left of the decimal
Rules for Scientific Notation
2. Determine “n” by counting
the number of places that the
decimal point was moved. If
the decimal was moved to the
left, n is positive; if the
decimal is moved to the right,
n is negative
Guided Practice
Express the following in
scientific notation:
1, 400, 000 km, the distance to
the sun
0.000 000 000 066 7, the
Universal Gravitational Constant
Independent Practice
Express the following in
scientific notation:
a. 85 000 000
b. 0.000 9
c. 74 000
d. 0.000 005
e. 30 000 000
f. 864 000
Express the following scientific
notations in the long form
1.
2.
3.
4.
5.
7 x 104
5.3 x 104
7 x 10-5
4.21 x 1010
47 000 x 105
1.
2.
3.
4.
5.
70 000
53 000
0.000 07
42 100 000 000
4 700 000 000
Express the following in
scientific notation
A. 500 008
B. 0.001 008 2
C. 456
Rules for Calculations Involving
Scientific Notation
1. Addition/Subtraction: In
order to add or subtract
numbers in scientific notation,
the numbers must be
expressed in the same
exponent or “n”
Rules for Calculations Involving Scientific
Notation
Example: 6.3 x104 + 2.1
x 105
1) Convert one number to
the same exponent
2) Add the number
Solution
6.3 x104 = 0.63 x 105
0.63 x 105
+ 2.1 x 105
2.73 x105
Rule for Multiplication
2. When multiplying, multiply
the coefficients and add the
exponents together (they do
NOT have to be in the same
power)
Rule for Multiplication
Example:
4 x 105 * 2 x 102
4 x 105 * 2 x 102 = 8 x 107
Rule for Division
When dividing, divide the
coefficients and subtract the
exponents (top exponent
minus bottom exponent)
Example: (4 x 105) / (2 x 107)
Ans:
2 x 10-2
Complete the following
calculations
1. (2.4 x 10-3) - (1.2 x 10-2)
2. (7.4 x 10-8) / (3.4 x 104)
3. ( 3.45 x 104) * (2.3 x 103)
Chemistry February 13, 2007
SI System
The SI system is a universal
system of measurements
based on powers of “10”
The US and Burma are the
only major countries that do
not use the SI system
SI System
The SI system used prefixes
“Kilo-” means 1000; symbol is k
“Centi-” means 0.01; symbol is c
“Milli-” means 0.001; symbol is m
“Deci-” means 0.1; symbol is d
Review-Chemistry
Copy and complete the chart in your notes:
Name of Prefix
Symbol Meaning
milli
c
1000
deci

Length
Length is the distance
between two points
The SI unit of length is the
meter (m)
Devices used to measure
length:
Rulers, Tape Measurers, and
Meter Sticks
Volume
Volume is the amount of
space an object occupies
Volume is length * width *
height
The SI unit for volume is m3
The liter (L) is also used for
volume of liquids
Volume Continued
Conversions for volume:
1 dm3 (decimeter cubed ) = 1 L
1 cm3 = 1 mL (milliliter)
Devices Used to Measure
Volume:
Ruler, Graduated Cylinder
Mass
Mass is the amount of
matter that an object
contains
Mass is measured in
kilograms (kg)
Grams (g) are also used
but are very small
Mass and Weight
Mass does not change
Weight does change
Weight is the force that the
Earth exhibits on a mass
Weight is measured in
Newtons (N)
Mass & Weight Continued
Question: How much mass
does a 60 kg person have on
the moon?
Answer: 60 kg
Question: How much does a
480 N object weigh on the
moon?
Answer: 80 N
Time
Time is the interval between
two events
Unit of time is the second
Temperature
Temperature is the
measure of the average
kinetic energy of
particles
Temperature measures
how hot or cold an
object is
Temperature
Temperature is
measured in Kelvin in
the SI system
K = 273 + °C
Temperature is usually
given in °C
Convert the following
temperatures
1.
2.
3.
4.
36 °C to K
50 K to °C
105 K to °C
-236 °C to K
Density
Density is the ratio of
mass to volume
Units for density include
g/cm3, g/mL, or kg/m3
Mathematically:
mass
m
D

volume v
Density Continued
Substances and their
densities:
Water
1.00 g/mL
Table Sugar
1.59 g/mL
Gold
19.3 g/mL
Ice
0.917 g/mL
Ethanol
0.789 g/mL
Density
1. What is the density of an
object that has a mass of 5
g and a volume of 2.5 cm3?
2. What is the mass of an
object with a density of 10
g/mL and a volume of 2
mL?
Demonstration of Density
OBJ: to see if different sodas
have different densities
We will use coke and diet coke
to see if the two sodas have
different densities
Question: Based on what was
observed, what can you
conclude about the density of
coke and diet coke?
Review-Copy and Answer in
Notes
1. What is density?
2. How much space does
5 g of a substance
occupy if it has a density
of 7.6 g / mL?
Significant Figures
Significant Figures (Sig
Figs): include all the numbers
that are known precisely plus
one last digit that is estimated
Significant Figures
Brainstorm: Think about
when you make a
measurement. Your reading is
based on the instrument that
you are using.
The last digit in a
measurement is an estimate
In a measurement, the last digit
is uncertain (meaning it is
estimated and not known exactly)
Question: What is the value in
having the last digit in a
measurement estimated?
By estimating the last digit, one
can obtain additional information
about the measurement
Activity for Sig Figs
Three students measure the length
of the board using 3 different meter
sticks. One of the meter sticks only
measures in units of 1. The second
meter stick measures in unit of 0.1.
The third meter stick measures in
units of 0.01.
Question: How precisely can each
student measure the board?
Activity for Sig Figs
The following measurements are
obtained:
Student 1: 3.5 m; Student 2: 3.70;
Student 3: 3.714 m
Activity for Sig Figs
1. Which student has the most
precise measurement?
2. If another student had to repeat
the measurements, which ruler
should the student use in order to
be precise?
3. Why did student 2 report the
value as 3.70?
Rules for Determining the
Amount of Significant Figures
In order to determine the
number of significant figures in
a measurement:
1. Every non-zero digit is
significant.
Example (Ex): 3.45 has 3 sig
figs
2. Zeros between non-zero
digits are significant
Ex: 5.091 has 4 sig figs (the zero
is between non zero digits)
3. Zeros appearing in front
of all non-zero digits are
NOT significant
Ex: 0.45 has only 2 sig figs
4. Zeros at the end of a
number and to the right of
a decimal point are
significant
Example: 3.40 has 3 sig figs
*The zero after a decimal point is
significant only if a non-zero digit
precedes it
5. Zeros at the end of a
measurement and to the left of
the decimal point are not
significant unless the zero was
measured
Example: 300 has only 1 sig fig
300.0 has 4 sig figs
Question
Explain why “300 m” has
only 1 sig fig but “300.0 m”
has 4 sig figs?
Consider the precision and
what type of ruler may
have been used to
determine these values
Determine the number of sig fig:
a. 10.5 g
a. 3
b. 112 mL
b. 3
c. 0.065 kg
c. 2
d. 2 527 cm
d. 4
e. 0.000 480 59 mg
e. 5
f. 12.000 m
f. 5
g. 1000 g
g. 1
Review
Determine the number of sig figs
in:
a. 456
b. 4.00
c. 6.090
d. 4000
e. 0.03650
Practice – Sig Figs
1) Visit Sig Fig Link
(home.carolina.rr.com/bwhitson)
2) Take Quiz at www.quia.com on
Sig Figs (individually)
Rules for Sig Figs in
Caculations
An answer cannot be more precise
than the least precise
measurement from which it was
calculated
Basically, if you have to do a
calculation with sig figs, your
answer cannot be more precise
than your least precise
measurement
Multiplication and Division
Multiplication and Division:
The number of significant
figures in the answer
(result) is the same as the
number of significant
figures in your least precise
measurement
Example Problem
Report 6.3 * 6.45 * 8.589 with
the correct amount of sig figs
Example Problem
1. Determine the amount of sig
figs in each number:
6.3 = 2; 6.45 = 3; 8.589 = 4
2. Calculate your answer
3. Report your answer to the
correct amount of sig figs
Solution
6.3 * 6.45 * 8.589= 349.01401
Your answer should have 2
sig figs
349.01401
First 2 Sig Figs
Ans) 350 or 3.5 x 102
Additional Practice
a.
b.
c.
d.
e.
2.7 / 5.27
4.5 * 9.56
2 * 5.6
4.00 * 2.658
89.5/ 2.0
Rules for Addition and
Subtraction
Addition and Subtraction:
The answer (result) has
the same number of
decimal places as the least
precise measurement
Example Problem
12.11 + 118.0 + 1.013
1. Determine which
measurement is least precise
2. Calculate
3. Place in correct number of
sig figs
Solution: 118.0 is the least
precise
12.11 + 118.0 + 1.013 = 31.123
Ans) 31.1
Practice
Report the following answers
with the correct amount of sig
figs:
1. 45.69 - 23.156
2. 12.35 + 47.360 + 12
3. 105.36 - 65.890
Percent Error (PE)
Percent Error is the
comparison of the actual (or
true) value to the
experimental
Essentially, it is the “percent
off” that you are from the
actual answer
Percent Error (PE)
Experimental Value is what
you obtained
True (Actual) Value is what
the true value is (correct
answer you should have
gotten)
Percent Error
PE =
Experimental  True( Actual)
True( Actual)
*100
Example Problems
1. You measure the mass of
an object to be 5.11 g. The
true mass is 5.20 g. What is
the percent error?
2. You find the value for  to
3.10. What is your percent
error?
Direct Relationship
Adsf
dsafd
Indirect (Inverse) Relationship
Adsf
dsafd