Atomic Masses

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Atomic Masses
How do you measure something too small to be seen?
A scale is needed what would be a good standard?
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C
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1 atomic mass unit (AMU) 1/12 mass of Carbon-12
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What would be the atomic mass of an element that
was twice as heavy as Carbon-12?
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What about one that is half as heavy?
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The average atomic mass is NOT the mass number
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- Gas state atoms are ionized at low pressure
- The ions are accelerated toward a magnet
- Magnet deflects them from straight line
- Deflection is inverse of atomic mass
- Atoms can be compared by weight
The Mass Spectrometer
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For example
It takes a magnet 1.583 times as strong to
deflect a stream of Fluorine atoms as it does a
stream of Carbon-12 atoms
What is the mass of Fluorine?
1.583 X 12.00amu 19.00amu
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Fluorine is unique
All Fluorine atoms have 9 protons and 10 neutrons
All other atoms have several isotopes
In order to determine the atomic mass The
relative amounts of each isotope must be
accounted for
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When a pure sample of Chlorine is passed through
the device the following graph is obtained
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What does this tell us?
Atomic mass
Abundance
Cl-35 34.97amu 75.53 Cl-37 36.97amu 24.47
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Atomic mass
- includes the fact that not all samples of
elements are like Fluorine
- not all abundances are like Chlorine
- the periodic table value is the average mass of
all of those elements in the universe
- the units are amus
Q How is the periodic table value calculated?
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What the?...
Y1 and Y2 are the masses of each isotope
Y1 and Y2 are the abundances of each isotope
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Bromine is a red-orange liquid that has an
average atomic mass of 79.90amu. Its name is
derived from the Greek bromos which means
stench. It has two naturally occurring
isotopes Br-79 (78.92 amu) and Br-81 (80.92
amu). What is the abundance of the heavier
isotope?
Strategy the two isotopes have to add up to
100. If Br-81 X, then Br-79 must equal?...
100 - X
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The heavier isotope has an abundance of 49
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Q If 75 of the Chlorine in the universe is
Cl-35 and the rest is Cl-37 should the table
entry for Chlorine be closer to 35 or 37?
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Q What is Chlorines atomic mass?
35.453 amu
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Masses of atoms and Avagadros number
Consider H and He Helium is about 4 times as
heavy as Hydrogen
A sample of 100 Helium atoms should weigh about 4
times as much as 100 Hydrogen atoms
A sample of a million Helium atoms should weight
about 4 time as much as a million Hydrogen atoms
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By that thinking 4 grams of Helium should
contain the same number of atoms as 1 gram of
Hydrogen
Specifically 4.003 grams of Helium contains the
same number of atoms as 1.008 grams of Hydrogen
A sample of any element with a mass in grams
equal to its atomic mass contains the same number
of atoms (NA), regardless of the identity of the
element
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The numerical value of NA is 6.022x1023 and is
known a Avagadros Number
It was obtained by experiments comparing the
weights of various gases whose relative weights
were already known
Q How many atoms are in a ten gram sample of
Titanium?
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Q How many atoms are in a ten gram sample of
Titanium?
10.0 g Ti
1 mole Ti
6.022 x 1023 atoms
x
x

48 g Ti
1 mole Ti
1.26 x 1023 atoms
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Avagadros Number worth of anything is called a
mole
The mass of one mole of a compound is numerically
equal to the sum of all of the atoms in the
formula
Sugar is C12H22O11
Baking Soda is NaHCO3
Copper nails are just Cu
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Mole Mass Conversions
m MM x n
One of the most important equations used in
chemistry!!!
(m) is the mass in grams
(MM) is the molar mass
(n) is the number of moles
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m MM x n
Aspirin is a compound called acetylsalicylic acid
(ASA). Its formula is C9H8O4 What is the mass of
0.509 moles of ASA?
m MM x n (180 g/mole) x (0.509 moles)
91.6 g
C 12 x 9 108 H 1 x 8 8 O 16 x 4 64
MM 180 g/mol
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Same problem done MY way
180 g C9H8O4
O.509 moles C9H8O4
x

91.6 g
1 mole C9H8O4
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Some problems are harder than others
A 1.00 g sample of aspirin contains 75.2 by mass
of C9H8O4 How many moles of ASA are in the sample?
Mass of ASA? 0.752 x 1.00g 0.752 g
1 mole C9H8O4
0.752 g C9H8O4
0.00418 moles
x

180 g C9H8O4
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How many Carbon atoms are present in a 12.0 g
sample of C9H8O4?
12.0 g C9H8O4
1 mole C9H8O4
6.02 x 1023 molecules
9 C atoms
x
x
x
180 g C9H8O4
1 mole C9H8O4
1 molecule
3.61 x 1023 Carbon atoms
There is only ONE problem in chemistry!!!