Metric System and Factor Label Problems

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Metric System and Factor Label Problems
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Units

• The system of units used in chemistry is the SI
  system (Systeme International)
• This is related to the metric system
• There are base units for length, mass, etc.
• There are decimal prefixes that modify the base
  unit

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What is the metric system?

• Used in nearly every country in the world
• Was devised by French scientists in the late 18th
  century to replace the chaotic collection of
  units then in use.
• The goal of this effort was to produce a system
  that uses decimals rather than fractions.

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What types of units are used for measurements?

• Mass - g, kg, mg
• Length - m
• Time - sec
• Temperature - K
• Volume - cubic cm (cm3)
• - liters
• - milliliters

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• Prefix Symbol Factor (see more pg 14)
• Giga G 109 1 Gm 109 m
• mega M 106 1 Mm 106 m
• Kilo k 103 1 Km 103 m
• Meter Liter Second Gram
• Deci d 10-1 10 dg 1 g
• Centi c 10-2 100 cg 1 g
• Milli m 10-3 1000 mg 1 g
• Micro µ 10-6 1000000 µg 1 g
• Nano n 10-9 10-9 ng 1 g
• Pico p 10-12 10-12 pg 1 g

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3 important calculations to have memorized

• Mass 1 pound 0.4536 kg (453.6 g)
• Volume 1 quart 0.9464 dm3 (0.9464 L)
• Length 1 inch 2.54 cm (exact)

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Temperature conversions

• There are three temperature scales that you need
  to be able to convert between

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Temperature and Heat are not the same thing

• Temperature
• A measure of the average kinetic energy of random
  motion of the particles in a substance
• Heat
• A measure of the total amount of energy
  transferred from an object of high temperature to
  one of low temperature

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Temperature

• Most commonly, we in the US use Fahrenheit to
  report temperature
• Where 32oF is freezing and 212oF is boiling
• But, most of the world measures temperature in
  Celsius
• Where 0oC is freezing and 100oC is boiling

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Converting Fahrenheit and Celsius

• oC 5
• ----- ( oF 32)
• 9
• oF 9
• ----- ( oC) 32
• 5

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Kelvin

• Many times, especially in working with gases,
  chemist use the Kelvin scale
• Where water freezes at 273.15 K and boils at
  373.15 K
• Absolute zero 0 K point at which all
  molecular motion ceases
• No negative numbers
• Convert from Celsius
• K oC 273.15

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Why do we need to know the metric system?

• We use the metric system numbers for conversion
  factors
• We use them to change from one unit into another
  unit

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Information to know for the metric system and
conversions

• Milli- 1m 1000 mm 1 g 1000 mg
  1 L 1000 mL
• Centi- 1 m 100 cm 1 g 100 cg
  1 L 100 cL
• Deci- 1 m 10 dm 1 g
  10 dg 1 L 10 dL
• Standard 1 m 1 meter 1 g 1 gram
  1 L 1 liter
• Kilo- 1000 m 1 km 1000 g
  1 kg 1000 L 1 kL

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Dimensional Analysis

• Method for setting up mathematical problems
• Also called factor label method
• Problems that give you one type of unit and ask
  you to convert to a different unit

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Example

• You are asked to find out how many seconds are in
  1 day.
• How would you go about solving this?
• Step 1 Always write out the given number and
  unit over 1 (make it a fraction)

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Example continued

• Step 2 We are trying to cancel the unit we have
  in order to get the one that is desired. So,
  list all of the conversion factors that you know
  about your given unit and decide which one you
  should use.

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Example Continued

• Step 3 Write your conversion factor(s) correctly
  so the units you do not want cancel out and you
  are left with your desired units

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Example finished

• Step 4 Once you have everything written down,
  you will multiply the numbers together.
• Multiply everything across the top. Then
  multiply everything across the bottom. Then
  divide.
• Get your mathematical answer and then round to
  sig. figs.

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Remember

• The units must cancel!
• Make sure that the answer is a reasonable one!

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Example

• Convert 2.3 miles to centimeters

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Example

• You mass a piece of iron in the lab. Its mass is
  1502 g. Lets say that you need to record the
  mass in kg, how would we do this?