Download answer key - El Camino College

ATOMIC WEIGHT AND ATOMIC MASS UNIT
(From College Chemistry by J.L. Rosenberg, L.M. Epstein, and P.J. Krieger)
1. What is atomic weight?
2. 12 grams of carbon-12 contain approximately 6.022×1023 atoms. What is the value of one
atomic mass unit (amu or u) in grams?
m 12 = 12 amu (exactly)
C
1 amu =
1
1
×m 12C =12 ×
12
12 g
6.022×10
23
= 1.66×10−24 g
3. Although there is only one naturally occurring isotope of iodine, 127I, the atomic mass of
iodine given in the modern periodic table is 126.90447, but not 127. Explain.
4. At one time there was a chemical atomic mass scale based on the assignment of the value
16.0000 to naturally occurring oxygen. What would have been the atomic weight, on such a
table, of silver?
mass of the “avg.” Ag atom on the old scale
mass of 16O atom on the old scale
X amu
16 amu (exactly)
=
=
107.8682 amu
15.9994 amu
mass of the “avg.” Ag atom on the new scale
mass of 16O atom on the new scale
A.W. of Ag on the old scale = 107.8722 amu
5. On the old atomic mass scale used by physicists, the mass of oxygen-16 atom was assigned
to be exactly 16 amu. What would be the atomic weight of magnesium on that scale?
mass of the “avg.” Mg atom on the old scale
mass of 16O atom on the old scale
X amu
16 amu (exactly)
=
24.3050 amu
15.9949 amu
=
mass of the “avg.” Mg atom on the new scale
mass of 16O atom on the new scale
A.W. of Mg on the old scale = 24.3127 amu
6. In a chemical atomic mass determination, the tin content in 3.7692 g of tin tetrachloride was
found to be 1.7170 g. If the atomic weight of chlorine is taken as 35.453 amu, what is the
value for the atomic weight of the tin isolated during this experiment?
mass of Cl in SnCl4 = 3.7692 g − 1.7170 g = 2.0522 g
Sn
Cl
=
X amu
4 × 35.453 amu
A.W. of Sn = 118.649 amu
=
1.7170 g
2.0522 g
X = 118.649
ATOMIC THEORY WORKSHEET
1. Which of the following statements of the atomic theory proposed by John Dalton at
the beginning of the 19th century are not quite true in light of modern atomic
physics? Rewrite each statement to reflect the current understanding of the atomic
theory.
An element is made up of atoms. All
atoms of a given element are identical.
Atoms cannot be created or destroyed.
All atoms of a given element have the
same number of protons in the nucleus.
They may differ in the number of
neutrons. Chemically the atoms of a
given element are virtually
indistinguishable: the types of chemical
reactions are the same; the rates may
slightly differ for different isotopes.
Atoms cannot be created or destroyed
in chemical reactions. In chemical
reactions the old bonds between atoms
are broken down and new bonds are
formed. Atoms, however, can be
created or destroyed in nuclear
reactions: radioactive decays, nuclear
fission and fusion.
All atoms of one element have the same
mass. Atoms of two different elements
have different masses.
Atoms of different elements may
combine in the ratio of small, whole
numbers to form compounds.
The main characteristic of an element is
the atomic number. Two elements differ
from each other by their atomic
numbers. In fact, atoms of two different
elements may have masses that are
very close to each other. For example,
39.9624 amu for an atom of argon-40
and 39.9640 amu for an atom of
potassium-40.
This is often the case for many
compounds, especially inorganic. It was
always the case for compounds known
to Dalton. Most organic compounds
(whose composition was not known to
Dalton) do not follow that rule: C12H22O11
(sugar), C27H46O (cholesterol),
C17H21NO4 (cocaine).
2. Calculate the atomic weight of neon using the data on its isotopes provided in the
table below.
Isotope
Number of
Protons
Number of
Neutrons
Mass
Number
Atomic Mass
(amu)
Natural
Abundance
(atom %)
20
Ne
10
10
20
19.9924
90.48
21
Ne
10
11
21
20.9938
0.27
22
Ne
10
12
22
21.9914
9.25
Weighted Average of Atomic Masses = 19.9924 amu × 0.9048 +
+ 20.9938 amu ×0.0027 + 21.9914 amu × 0.0925 = 20.18 amu = Atomic Weight or
Relative Atomic Mass (the number below the symbol in the Periodic Table of the
Elements)
ATOMS, ISOTOPES, ELEMENTS
Symbol
of the
Atom
(Isotope)
Atomic Mass
Number Number
Number
of
Protons
in the
Atom
Number of
Neutrons
in the
Atom
Number of
Electrons Mass of
the Atom
in the
Atom
Name of
the Element
Name of
the Isotope
235
92
92
235
92
143
92
235.0439
amu
uranium
uranium-235
238
92
U
92
238
92
146
92
238.0508
amu
uranium
uranium-238
34
16
S
16
34
16
18
16
33.9679
amu
sulfur
sulfur-34
37
17
Cl
17
37
17
20
17
36.9659
amu
chlorine
chlorine-37
39
19
K
19
39
19
20
19
38.9637
amu
potassium
potassium39
12
6
C
6
12
6
6
6
12 amu
(exactly)
carbon
carbon-12
13
6
C
6
13
6
7
6
13.0034
amu
carbon
carbon-13
Na
11
23
11
12
11
22.9898
amu
sodium
sodium-23
U
23
11