Atomic Masses

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Atomic Masses

• Using proportions in which elements combine,
  scientists determined relative atomic masses

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Atomic Masses

• Picked the element carbon to be standard
• 12C is assigned a mass of exactly 12 atomic mass
  units

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Atomic Masses

• Mass spectrometer- used to compare masses of
  atoms by forming positive ions and then shooting
  them through a magnetic field

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Atomic Masses

• The amount of path deflection depends on an
  atoms mass
• Example when 12C and 13C are analyzed, their
  mass ratio is 1.0836129

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Atomic Masses

• Mass of 13C (1.0836129) x (12 amu) 13.
  003355amu
• So, if 12C is our standard why is its atomic mass
  12.01 and not 12.00?

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Atomic Masses

• The law of averages!!!
• The atomic mass on the periodic table is an
  average of all the isotopes of that element based
  on the percentage in an avg sample

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Ave Atomic Mass Is?

• An element consists of 90.51 with a mass of
  19.992 amu, .27 of an isotope with a mass of
  20.994 amu, and 9.22 of an isotope with a mass
  of 21.990 amu.

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Atomic Mass Soln

• (.9051)(19.992) 18.09
• (.0027)(20.994) .057
• (.0922)(21.990) 2.03
• Sum of above 20.18

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M M Problem

• What would be ways for me to calculate for each
  of you 57 M Ms?
• Counting
• Weighing

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M M Problem

• If we found the average mass of an M M, we
  could then weight out very close to 57 M Ms
  quickly.
• No M Ms may have exact mass- only need average

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Every Chemists Favorite Unit

• Samples of matter usually contain lots of atoms
• Our standard is based on how many atoms are in
  12 grams of pure 12C

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Every Chemists Favorite Unit

• This unit has a very special name that you will
  all learn to love dearly It is called

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Every Chemists Favorite Unit
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Every Chemists Favorite Unit

• In 12 grams of 12C there are 6.02 x 1023 atoms
• This is our definition of the MOLE!!

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Some Mole Facts...

• 1 mole of seconds is 4 million times as long as
  the earth has existed!!!
• 1 mole of marbles would cover the entire earth 50
  miles deep!!!

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Some Mole Analogies

• A mole is much like using the word dozen
• We know dozen means 12
• We know mole means 6.02 x 1023

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Some Mole Problems!

• You have a gold ring that is 10.94g.
• How many moles is this?
• How many atoms is this?

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Some Mole Problems!

• A sample contains 5.00 x 1020 particles of iron.
• What fraction of a mole is this?
• What is the mass of this?

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Molar Mass Is...

• Mass in grams of one mole of a COMPOUND!
• Term also used is molecular weight

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Molar Mass

• To find the molar mass of a compound, sum the
  masses of the component elements

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Molar Mass Problems

• Calculate the molar mass of acetic acid.
• What is the formula?

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Molar Mass Problems

• You are given 89.72g of acetic acid.
• How many molecules of acetic acid are there?
• How many C atoms are there?