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CHEMISTRY
August 8, 2011
CHEM BRAIN SAMPLER
August 8, 2011
Your group has 5 minutes!!
Place homework on your desk
1 . Why do we have an SI (system International)
system for units chemistry?
2. How many significant figures are in:
0.00236
2.030
50.00
102,000
3. What is the SI unit for time? mass? temperature?
4. Refer to the diagram on the board. Report the
measurement.
(include an appropriate unit)
Significant Figures
Calculations
August 8, 2011
Significant Figures in Calculations


What are
Significant
Digits?
Examples
 Triple
Beam
Balance
 Graduated
Cylinder


All the certain digits plus the
estimated digit in a measurement.
How many decimal places can we
count
Significant Figures in Calculations


Multiplication
and Division

The number with the smallest
number of significant digits
determines how many significant
digits are allowed in the final
answer.
Example

Volume of a box
L
xWxH
 (3.05m)(2.10m)(0.75m)
 2 sig figs
 4.8m3
Significant Figures in Calculations

D
Example
 Density
of a
penny
 M = 2.53g
V = 0.3mL
=M/V
 # significant figures allowed
 D = 8g/mL
Significant Figures in Calculations


Addition and
Subtraction
Example
The number of significant digits
depends on the number with the
largest uncertainty. (you may be
using different scales)
Shoes
951.0 g
Clothes 1407
g
Ring
23.911 g
Glasses 158.18 g
Total
2540.
g

Significant Figures in Calculations

Example

What is the mass of a penny if,
the weighing paper alone has a
mass 0.67 g and weighing
paper plus the penny has a
mass of 3.2 g.
3.2 g
-0.67 g
2.5 g
Significant Figures in Calculations

Remember
A calculated number can only be as
precise as the least precise
measurement in the calculation.
Practice
Calculate each of the following to the correct number of
significant figures. Include units on your answer.
1.
(25 g/mol)(4.0 mol) =
2.
(3.48 in)(1.28 in)(0.010 in) =
3.
2.06 cm + 1.8 cm + 0.004 cm =
4.
If the mass of a lead cube is 176.91 g and it measures
2.51cm x 2.49 cm x 2.49 cm, what is the density of
lead?
Practice
Calculate each of the following to the correct number
of significant figures. Include units on your answer.
1.
(25 g/mol)(4.0 mol) =1.0 x 102
2.
(3.48 in)(1.28 in)(0.010 in) = .045 in3
3.
2.06 cm + 1.8 cm + 0.004 cm = 3.9 cm
4.
If the mass of a lead cube is 176.91 g and it measures
2.51cm x 2.49 cm x 2.49 cm, what is the density of
lead? 11.3 g/cm3
Dimensional Analysis
August 8, 2011
Dimensional Analysis
Dimensional
Analysis
Example

A tool. An organized
technique of converting
between units.
How many eggs are in two
cases?
1 case = 24 cartons
1 carton = 12 eggs

Dimensional Analysis
Equivalence
Statement


Examples
Conversion
Factors
Examples
Relates the same quantity in two different
units
An equation that is always equal.
1 gallon = 3.785 L
2.54 cm = 1 inch

Ratio between units in the equivalence
statement
1 _ gallon
3.785L
or
3.785L
1 _ gallon
Dimensional Analysis

How to use
Dimensional
Analysis (DA)
1.
2.
3.
4.
5.
6.
Identify the unknown. What is the
problem asking you to find?
Start with the given (often the first #
in the problem)
Identify and set up the conversion
factors for each unit of change
Make sure all units cancel EXCEPT
the one you are looking for.
Check that you have the correct
number of significant figures.
Ask whether your answer makes
sense.
Brain Teaser



Usain Bolt, a Jamaican Olympian, just ran a world
record for a 100 meter dash sprint. He ran this race
in 9.68 seconds beating the world record by 1-one
hundredth of a second (9.69s).
How fast was he running?
How fast was he running in miles per hour?
1 mile = 1609.34 meter
 60 seconds = 1 minute
 60 minutes = 1 hour

Sample Problems
1.
2.
3.
4.
How many kilograms are in 125 g of salt?
Chloroform is a liquid once used as and
anesthetic. What is the volume in mL of 5.0 g
of chloroform. Density = 1.49 g/mL?
The length of a marathon race is
approximately 26.2 miles. What is the
distance of the race in feet?
5280. feet = 1 mile
Convert the freeway speed limit of 65 miles
per hour to kilometers per second.
1 km = 0.62 mi

Start with the given, end with what you need!
Practice
1. How many pieces of paper are in 3 boxes of
copy paper if each box contains 16 reams and
each ream is 500 pieces of paper?
TEAM
The Perfect Cup of Kool-Aid
It is a hot summer day and you are extremely thirsty. You
decide that a cup of Kool-aid would be perfect to quench
your thirst. You find the following recipe for the perfect
cup of Kool-aid:
 0.05
cups of Kool-aid powder should be mixed with 7.5 fluid
ounces of water.
However, in your kitchen you only have teaspoon with which
to measure. Using dimensional analysis and the given
equivalence statements, determine the number of teaspoons
of Kool-aid and water you need to make your drink.
5 mL = 1 tsp
17 Tbsp = 1 cup
1 Tbsp = 15 mL
8 fl oz = 1 cup
Summary



What is dimensional analysis used for?
Why is it useful?
What must accompany every number
(measurement) in chemistry?