Chapter 2 Notes
Qualitative/Quantitative
Precision/Accuracy
Sig Figs
Error/Percent Error
What do you know?
1. What is a hypothesis?
2. What is the difference between an
control and a variable?
3. What is the difference between an
independent and dependent variable?
Experiments
System: Matter that you’ve chosen to
observe and the region of space
selected for study.
Surroundings: the area around the
system.
Demo
Draw a picture of what happened!
Write a couple of sentences regarding
what we will discuss.
What is the System?
Surroundings?
MeasurementsWhy do you think measurements might
be important to the study of chemistry?
What kind of measurements are you
familiar with?
Day 1 of Measurement Activity
Splash of Color-
Accuracy and Precision Activity-
Group Work
Everybody is on task and working at all times.
Everyone is responsible for handing in a
completed assignment!
Jobs:
1. Leader- Reads all the instructions (out loud) and
keeps everyone on task
2. Scientist 1- Reviews instructions and does the
task.
3. Recorder- Keeps careful record of all
observations to share with other group members.
Debrief the Activities:
1. What is the difference between
quantitative and qualitative
observation?
2. Which is most helpful to scientists?
3. What is the problem with using
qualitative measures?
Debrief
4. Was it a chemical or physical change?
Types of Measurements
Qualitativedescriptions
no numbers
determined using senses
vary depending on observer
Quantitative
more definite
use numbers (with units)
only as good as the instrument used
Quantitative Measure
Quantity- a measure of magnitude, size or
amount.
Units- “Le Systeme International d’Unites.
Adopted in 1960- agreed on world-wide
7 base units
SI Base Units
Quantity
Symbol Name
abbreviation
Length
l
Meter
m
Mass
m
kilogram
kg
Time
t
Second
s
Temperature
T
Kelvin
K
Amount of
substance
n
Mole
mol
Non-Si Units Used in
Chemistry
Physical Quantity Unit Name
Symbol
Volume
Pressure
L
Atm
Energy
Liter
Atmosphere
Millimeters of
mercury
Torr
Calorie
Joule
mmHg
Torr
Cal
J
Derived SI Units
You already know two of these from
Math!
Area
What formula would
you use to
determine the area
of this rectangle?
What would the units
be?
Volume
What formula would
you use to
determine the
volume of this box?
What would the units
be?
Using a Graduated Cylinder
Another way of
determining volume
is using this
fabulous tool.
1ml = 1cm3
Always keep track of your Derived SI
Units in any calculation!
Quantity
Unit
Abbreviation
Derivation
Area
Meter squared
m2
LxW
Volume
Meter cubed
m3
LxWxH
Density
Kilograms per
cubic meter
Kg/ m3
Mass/volume
Molar Mass
Kilograms/mol
Kg/mol
Mass/amt of
substance
Density
What formula do we
use to calculate
density.
What do we need to
measure?
Density
Density = Mass/Volume
Density is a useful way of
determining the identity of
a substance!
Is it a chemical or physical
property?
Try a practice problem!!
A sample of aluminum metal has a mass
of 8.4g. The volume of the sample is
3.1 cm3. Calculate the density.
What is the difference between
accuracy and precision?
The Accuracy/Precision Activity looks at
How “Good” is the measurement?
Accuracy- how close
are we to the real
value?
Precisionhow reproducible
are the results?
Were your measurements in the
activity accurate, precise or both.
Why was it important to use the same
measuring tools for each trial?
How could you have gotten better results?
Accurate or Precise??
Let’s look at some data!
Accurate? Precise?
Shadows’ weight
(experimental)
62.7 lbs
62.0 lbs
62.5 lbs
Her weight according
to the vet (accepted)
67.4 lbs
Miles to school
(experimental)
3.9 miles
3.8 miles
3.9 miles
Mapquest (accepted)
3.98
In Our Dream Life—
All our measurements would be BOTH
Accurate and Precise!
Scientists strive for correct measurements
that are reproducible.
With No Error!
Experimental Measurements
are seldom perfect
We often compare experimental values to
known, widely accepted values and
calculate the error.
You learned about this during the
measurement activity!!
Error
Experimental value is measured in the lab
Accepted value is based on reliable
references
Error = lAccepted - Experimentall
What formula did you use to
calculate percent error?
Percent error is an indication of how well
the lab went… if you have a low percent
error, it means that there were few lab
errors.
Calculate the percent error!
Shadows’ weight
(experimental)
62.7 lbs
62.0 lbs
62.5 lbs
Her weight according
to the vet (accepted)
67.4 lbs
Miles to school
(experimental)
3.9 miles
3.8 miles
3.9 miles
Mapquest (accepted)
3.98
Percent Error
What are some lab mistakes you could
have made that would effect your
accuracy and precision?
Gen Chem- Homework… percent error WS.
Warm Up: How did you do on
%Error WS?
Front Side
1. 4.62%
2. 24.3%
3. 12%
4. 0.9%
5. 0.3%
Back Side
1. 5.6%
2. 2.6%
3. 1.8%
4. 10.%
5. 0.519 mL
6. 5.6%
Warm Up- Friday
1. What is the density of a substance with a length of
3.5 cm a width of 2.1cm and a height of 4.5 cm and
a mass of 261 g. What is the substance?
Known Densities
Zinc
Iron
Copper
Gold
7.14 g/cm3
7.89 g/cm3
8.92 g/cm3
19.39 g/cm3
2. The density of aluminum is 2.7g/cm3. What mass of
aluminum takes up 1.2 cm3 of space?
Warm Up Answers
1. Iron
2. 3.24 g
Work on the density WS… we will fix your
answers for SF on Monday
Measurement Activity- 3 and 4
Same groups as previous class.
Change jobs!
1. Leader- Reads all the instructions (out loud) and keeps
everyone on task
2. Scientist 1- Reviews instructions and does the task.
3. Recorder- Keeps careful record of all observations to share
with other group members.
You have 25 minutes to complete
these two tasks!
Warm Up- Monday/Tuesday
How would you determine the volume of
an irregular shaped object?
General Chem
Collect the “Why I need to study
Chemistry” assignment.
Debrief Activities
1. So, what is a significant figure?
2. Why is it important that we use them?
3. How hard was it to “estimate” the last
digit on the ruler? How about the
beaker?
Sig Figs
Significant figures include all the known
digits plus a last digit that is estimated.
Significant Figures
Depends on the instrument being usedThe better the instrument
(read more expensive)
the more significant figures.
Which was a better measuring tool?
The beaker, the cylinder or the volumetric flask?
Rules for Zeros:
1. All non zero digits
2. Zero’s in between digits
3. Zero’s in front are not sig
4. Zero’s in back are not sig unless it is a
known measured value (there is a decimal
point).
5. Unlimited sig figs
When something is counted
Exactly defined quantities (constants)
Every calculation in this class, you
need to look at the Sig Figs.
When you multiply or divide: count the SF
and round your answer to the
lowest number of SF.
The rule is a little different
when you add and subtract…
Do the addition or subtraction…
Look at the numbers and round to
the measurement with the least number
of decimal places.
Rounding
If the number after the last sig fig is 5 or
greater– round up.
If the number after the last sig fig is less
than 5– keep it the same!
Look at the Sig Fig WS.
Do a few of these… yes, you can use your
calculator!
Get the answer… and round your answer
to the correct number of SF.
How did you do on %Error
WS?
Front Side
1. 4.62%
2. 24.3%
3. 12%
4. 0.9%
5. 0.3%
Back Side
1. 5.6%
2. 2.6%
3. 1.8%
4. 10.%
5. 0.519 mL
6. 5.6%
What about the SF???
How did you do on the
Density WS?
Check your significant
How do I do conversions
Using Dimensional Analysis