Name
Date
Period
Atomic Mass Problems
The object of this exercise is to develop the ability of working atomic mass problems. Express
decimal places, Show all work and include any units.
all
answers
to
two
Example: There are two naturally occurring isotopes of silver: Ag-107 and Ag-109. The relaüve abundance of Ag-107
51.84 and Ag-109
the
is
46.16. Find average atomic mass.
numbers together to
10.81 amu
1.
find the
to regular
weighed average. (0.5184 x 107) + (0.4816 x 109) = 107.96
amu
There are two naturally occurring isotopes of Boron: Boron-10 and Boron -11.
abundance of Boron-10
is
19.91% and boron-11
is
80.09%, what
is
is
number then multiply by the mass and add
the average atomic
mass
of
if
the relative
Boron?
(10)(19.91) + (11)(80.09) / 100 = 10.8009
35.48 amu
2.
There are
abundance of Chlorine-35
is
naturally occurring isotopes of Chlorine: Chlorine-35
and Chlorine-37.
if
the relative
75.78% and Chlorine-37 is 24.22%, what is the average atomic mass of Chlorine?
(35)(75.78) + (37)(24.22) / 100 = 35.4844
63.96 amu
relative
There are Wvo naturally occurring isotopes of Copper: Copper-63 and Copper-65. Ifthe
3.
abuhdance of Copper-63
is
69.17% and Copper-65
is
30.83%, what is the atomic mass of Copper?
(63)(69.71) + (65)(30.83) / 100 = 63.9568
69.80 amu
and Gallium-71.
4. There are two naturally occurring isotopes of Gallium: Gallium-69
abundance of Gallium-69
js
60.11% and Gailium-71
is
39,890/0,
Ifthe relative
what is the atomic mass of Gallium?
(69)(60.11) + (71)(39.89) / 100 = 69.7978
186.25 amu
5.
atomic mass.
121.86 amu
and Sb-123
Rhenium
exists as
naturally occurring isotope:
37.4% Re-185 and 62.6% Re-187. Find average
(185)(37.4) + (187)(62.6) / 100 = 186.252
6. There are two naturally occurring isotopes of Antimony.
is
42.79, what is the average atomic
If
the relative abundance. of Sb-121
is
5721
mass?
(121)(57.21) + (123)(42.79) / 100 = 121.8558
85.56 amu
7. Rubidium
is
found with the following composition: 72.17% Rb-85 and 27.83% Rb-87. Find average
atomic mass.
(85)(72.17) + (87)(27.83) / 100 = 85.5566
192 amu
8. Find the average atomic
mass ifthere is 37.3%
Ir-191
and 62.7%
IF-193.
(191)(37.3) + (193)(62.7) / 100 = 192.254
21.7 amu
26.
9.
Magnesium
is
found with the following composition: 78.9% Mg-24, 10.00% Mg-25, and 11.01
% Mg-
What is the average atomic mass?
((24)(78.9) + (25)(11.01)) / 100 = 21.6885
28.11 amu
3.09%
10. Silicon exists in nature as three isotopes:
92.23%
silicon-28,
4.68%
silicon-29,
and
silicon-30.
((28)(92.23) + (29)(4.68) + (30)(3.09)) / 100 = 28.1086
65.47 amu
11. Zinc exists in nature as five isotopes. Zn-64 occurs
48.63% Zn-66 occurs 27.90%, Zn-67 occurs
4.10%, Zn-68 occurs 18.75% and Zn„70 occurs 0.62%. What is the atomic mass of Zinc?
((64)(48.63) + (66)(27.9) + (67)(4.10) + (68)(18.75) + (70)(0.62)) / 100 = 65.4682