Transcript Formula to calculate avg atomic mass

Average Atomic Mass
Chemistry Notes
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Relative Atomic Mass
 Masses of atoms expressed in grams
are very small, for example: One
atom of Oxygen-16 has a mass of
2.66x10-23g =
0.0000000000000000000000266g
 The standard used to denote units of
atomic mass is the carbon-12 nuclide.
 It has been arbitrarily assigned a
mass of exactly 12 atomic mass units
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Relative Atomic Mass
 One atomic mass unit, or amu, is
exactly 1/12 of a carbon-12 atom.
 Atomic Mass is determined by
comparing it with the mass of a
carbon-12 atom.
 Therefore, atomic mass is a measure
of an atom’s mass relative to carbon12.
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Average Atomic Mass
 Most elements occur naturally as a mixture
of isotopes.
 The percentage at which each isotope
occurs in nature is taken into account when
calculating the element’s average atomic
mass.
 Average Atomic Mass- the weighted
average of the atomic masses of the
naturally occurring isotopes of that
element.
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 Compare average atomic mass to
mass number
 Mass number is the sum of the protons
and neutrons of ONE isotope of an
element (whole number)
 Average atomic mass is the weighted
average of ALL of the isotopes of an
element (usually not a whole number b/c
its an average)
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How Do You Calculate a
Weighted Average?
 You have a box of 100 marbles.
 25 of the marbles have a mass of
2.0g each.
 75 of the marbles have a mass of
3.0g each.
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Weighted Average Calculation
 Calculate the total mass of the
mixture & divide by 100.
25 x 2.0g = 50g
75 x 3.0g = 225g
 Total mass = 275g
 275g ÷100 = 2.75g (average mass of
marbles)
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Calculate the Average Atomic Mass
of an Element (amu)
Isotope
Mass
Number
% Natural
Abundance
Atomic
Mass
(amu)
Copper-63
63
69.17%
62.93
(69.17 copper
atoms out of
100 are this
isotope)
Copper-65
65
30.83%
64.93
(30.83 out of
100 are this
isotope)
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Calculate the Average Atomic Mass
of an Element (amu)
 Copper-63
69.17 x 62.93 amu = 4353 amu
 Copper-65
30.83 x 64.93 amu = 2002 amu
 Total Mass = 6355 amu
 6355 amu ÷ 100 = 63.55 amu
 What is Copper’s Atomic Mass on the
periodic table?
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Calculate the Average Atomic Mass
of Oxygen
Isotope
Mass
Number
% Natural
Atomic
Mass
Abundance
Oxygen-16
16
99.76
15.995
Oxygen-17
17
0.038
16.999
Oxygen-18
18
0.200
17.999
(amu)
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Answer
 16.00 amu
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Formula to calculate avg atomic
mass
 Multiply the mass of each isotope by its
percent abundance and then add them all
together; then divide the total by 100
(Mass of isotope 1 x % abundance) + (mass of isotope 2 x % abundance)+
100
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Which isotope is most abundant?
 The average atomic mass of an element is
always closest in value to the most
abundant isotope.
 For example, Carbon’s average atomic mass is
12.011, so the most abundant isotope of carbon
is carbon-12.
 The average atomic mass of sulfur is 32.06. The
isotopes of sulfur are sulfur-32, sulfur-33, sulfur
-34, and sulfur-36. What is the most abundant
isotope of sulfur?
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 Sulfur-32
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Assignment
 Pg 117 practice problems #23-24
 Pg 124 problem #81
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