Classwork:
Period 3:
Introduction to unit analysis
Units of Measurement
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All measurements must include both a unit and a number.
o Without the unit, the number
has no meaning.
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English vs. Metric System:
o English system – feet,
inches, etc are not used in science.
o Metric system- the international
system of measurement is used
§ Common
language for all scientists
§ Easy
conversions
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SI Base Unit |
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Physical Quantity |
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Unit Name and
Symbol |
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mass |
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kilogram, kg |
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length |
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meter, m |
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time |
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second, s |
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count, quantity |
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mole, mol |
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temperature |
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kelvin, K |
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electric current |
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ampere, A |
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luminous intensity |
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candela, cd |
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Derived Units
Commonly Used in Chemistry |
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Physical Quantity |
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Unit Name and
Symbol |
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area |
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square meter |
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volume |
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cubic meter |
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force |
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newton, N |
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pressure |
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pascal, Pa |
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energy |
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joule, J |
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power |
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watt, W |
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voltage |
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volt, V |
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frequency |
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hertz, Hz |
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electric charge |
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coulomb, C |
The International System of Units (SI)
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Seven base units (shown above)
Definitions:
o Length: distance
that light travels in a vacuum during a time interval 1/299,792,458 of a second
o Mass and weight:
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Mass: amount of material- about 2.2 lbs at sea level
§
Weight: influence of the force of gravity on mass
o Area and Volume
(derived units – combinations of base units)
§
Area = Length x Width
5.0 m x 3.0 m = 15 m2
· Both
units and numbers are multiplied in the answer
§
Volume: amount of space that an object occupies
Non- SI Units Used Frequently in Chemistry
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Volume: liter, L (there are exactly 1000 L in one cubic meter)
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Pressure: atmosphere, atm; millimeters of mercury, mm Hg
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Temperature: Celcius degree
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Energy: calorie, cal
Metric Prefixes
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Prefixes added to the base unit that make the units larger or smaller
Prefixes that make the Unit Larger
o kilo (1 km =
1000 m)
o mega (1Mm = 1000000
m)
Prefixes that make the Unit Smaller
o deci (1 dm =
.1 m or 10 dm = 1m)
o centi (1cm =
.01m)
o milli (1mm =
.001 m)
o micro (1mm
= .000001 m)
o nano (1 nm =
.000000001 m)
o pico (1pm .000000000001
m)
Four Step Problem Solving Strategy:
- Analyze –
read problem, identify the unknown quantities. Organize information into a table
or list. Sketch picture or diagram to help.
- Plan –
is the problem similar to previous problems? Write down any equations that link
the unknown and given information. Estimate and ask yourself, does the answer
seem reasonable.
- Solve –
perform calculations. Make sure to check units and significant digits.
- Evaluate –
does the answer make sense. Compare the answer to the estimate.
Unit Equations and Unit Factors
Based on equivalent relationships
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statement of a relationship between two quantities that are equal
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will be used during unit conversions
Example: 1 dime = 10 pennies
Unit Equations:
Is a series of two equivalent quantities
Ex: 1 dime = 10 pennies
10 pennies = 1 dime
Unit Factor:
Ratio of two equivalent quantities
Ex: 1dime/ 10 pennies or 10 pennies/ 1 dime
Both the quantity and the reciprocal are true.
Exactly Equivalent Equations:
These equations are equivalent by definition, such
as 1 foot is equal to exactly 12 inches.
As a result, rules for significant figures do not apply to
these quantities are not considered when rounding for significant figures when doing the calculation.
A three lined equal sign is used to express these quantities.
Unit Analysis:
Also known as dimensional analysis or the factor label method.
A simple three step process:
Step 1:
Read the problem, determine the units
needed in the answer
Step 2:
Read the problem, determine which measurements given relate
to the answer.
Step 3:
Use Unit Factors and exact equivalents to convert units
through the equation to reach the desired answer units.
Period 4:
no class
